How vulnerable is the seat?

• Inside Elections: toss-up
• Cook Political Report: toss-up
• Sabato's Crystal Ball: toss-up

Why is it in play?

Call it an inspired speech or a face-saving bow-out after a 33 percent approval rating, but Jeff Flake’s party condemnation and campaign suspension in October has left a power vacuum in Arizona. Though the president has only a 44.2 percent approval rating in the state, Flake’s long-standing criticism of the president was enough to doom him and unsettle the local GOP Establishment, who now have to find a measured head to run against the freewheelin’ Trump-era candidate Kelli Ward. There hasn’t been a Democratic senator from the deep-red state in over 20 years, but a close presidential race, bipartisan representation in the House, and Flake’s contested 2012 win signal an opportunity for Dems to chaos-ladder their way to victory.

Who are the challengers?

Republican congresswoman Martha McSally — who did not endorse Trump and has criticized his language toward “veterans and Hispanics and women and others” — has also all but formally announced a bid, which would make her the Establishment front-runner. Republican Kelli Ward ate a 12.5-point loss in her primary long shot against John McCain, but faces better odds running for the junior seat, with $300,000 from Robert Mercer, and endorsements from Steve Bannon and Rand Paul. She calls herself a “build-the-wall, stop-illegal-immigration Americanist,” suggested John McCain resign “as quickly as possible” after his brain-tumor diagnosis, and would like the U.S. to leave the U.N. But Arizona party leaders worry that her conservative-populist bona fides could hurt their chances in the general election. As a primary alternative, the party has its hopes in potential candidates such as Arizona treasurer Jeff DeWit, the Trump campaign’s chief operating officer, who would surely enjoy presidential backing if he made good on his reported interest in the seat.Centrist and Democratic congresswoman Kyrsten Sinema is the front-runner on the left. The first openly bisexual person in Congress, Sinema is a member of the conservative-leaning Blue Dog Coalition of Democrats, and has moved away from her party on key issues — in 2015 and 2017, she did not support Nancy Pelosi as Speaker of the House and was one of only seven House Democrats to approve the Select Committee on Benghazi. As of September, she had$4.17 million on hand to fund her bid. Currently, she has no major primary opponents, which could change if astronaut Mark Kelly listens to party encouragement and joins the race. The husband of former congresswoman Gabrielle Giffords, Kelly was thrust into the national spotlight after his wife was the target of an attempted assassination in 2011. In the past, he’s been called the“Democrats’ dream candidate.”

How vulnerable is the seat?

• Inside Elections: tilt democratic
• Cook Political Report: lean democratic
• Sabato's Crystal Ball: toss-up

Why is it in play?

As Florida’s only statewide elected Democrat, the 75-year-old, third-term senator is solidly centrist. He’s also never faced serious competition for his Senate seat, but he may this year if Florida’s popular governor, Rick Scott, who is term-limited, enters the Senate race, which he’s widely expected to do. An October poll showed Scott and Nelson neck and neck, pointing to an expensive race ahead. Both Democratic insiders and grassroots activists worry Nelson is out of touch with the party’s current trends. Nelson appealed to national Democratic leadership for help over the summer, and his race will likely get extra financial help from the party. However, there’s some reason to think Nelson will hang on — an analysis from the University of Minnesota found that in the 17 Senate elections since 1966, Floridians chose a nominee from the president’s party only five times, or less than 30 percent of the time.

Who are the challengers?

Governor Rick Scott would be a formidable challenger. His approval rating has hovered around 54 percent. He also has the wealth to fund his campaigns, and has spent at least $86 million on his own gubernatorial campaigns in the past. And while he hasn’t yet declared his candidacy, Democrats have already begun targeting him with TV ads and voter-registration drives. The only Republican to declare his or her candidacy so far is white supremacist Augustus Sol Invictus, who was recently accused of drinking the blood of a sacrificed goat and trying to lure neo-Nazis into the Libertarian Party. Nelson might also face a bruising primary: Law-school professor Tim Canova, who lost a bid to unseat Congresswoman Debbie Wasserman Schultz last year, and Orlando state senator Randolph Bracy have said they’re considering bids.

How vulnerable is the seat?

• Inside Elections: toss-up
• Cook Political Report: toss-up
• Sabato's Crystal Ball: toss-up

Why is it in play?

Like many of his fellow Senate Democrats from states won by Trump, Donnelly is a moderate often at odds with his own party. He’s pro-life, didn’t endorse same-sex marriage until 2013, and opposed Obama’s 2014 executive orders regarding immigration. Donnelly was also one of three Democrats who voted for Supreme Court nominee Neil Gorsuch. He’s the only Democrat elected statewide in Indiana, and his 2012 win was largely attributed to his opponents’ many mistakes. Outside groups have already run more than $2.5 million in ads against him this year. But with a 47 percent approval rating and more than $4 million already in the bank, Donnelly could be doing worse.

Who are the challengers?

Things have gotten brutal between the Republican front-runners, U.S. representatives Luke Messer and Todd Rokita, leading Politico to dub it the “GOP’s nastiest primary.” Rokita has referred to Messer as “unhinged,” and Messer’s Wikipedia entry was edited to mimic Rokita campaign rhetoric. It’s also causing divisions between Pence, whose brother supports Messer, and Trump, whose prominent supporters are backing Rokita. Messer's fundraising edge over Rokita has been slipping –– Rokita has closed the gap to within $50,000. Rokita has also served two terms as secretary of state and has experience winning statewide campaigns. Either Messer or Rokita would prove a formidable challenger against Donnelly, but the ideologically similar congressmen will face a brutal primary. Three less-well-known Republicans have also entered the race, including Mark Hurt, a lawyer from the small city of Kokomo; Andrew Takami, a university administrator; and Terry Henderson, a businessman from an Indianapolis suburb.

How vulnerable is the seat?

• Inside Elections: likely democratic
• Cook Political Report: toss-up
• Sabato's Crystal Ball: leans democratic

Why is it in play?

Second-term senator Al Franken resigned in mid-December after eight women separately accused him of groping and forcibly kissing them. Franken hadn’t been up for reelection until 2020, and Democrats hadn’t anticipated defending the seat heading into 2018. Minnesota governor Mark Dayton appointed Lieutenant Governor Tina Smith to replace him, and she’ll have to defend the seat next year (and again in 2020). A rising political star in the state, Smith has been lieutenant governor since 2015, and served as the governor’s chief of staff before that. Though Minnesota leans Democratic, Republicans control both chambers of the state legislature, and Clinton won the state by a much narrower margin than expected. Franken’s resignation also sets the stage for massive upheaval in the state next year: Both Senate seats and the governor’s seat will be in play.

Who are the challengers?

No one has officially declared his or her intent to challenge Smith next year, but observers are asking whether Republican candidates for the governor’s seat (like the state’s former GOP chair, Keith Downey) might switch races. Representative Keith Ellison has said he will not challenge Smith for the Senate next year, decreasing her chances of a fraught primary.

How vulnerable is the seat?

• Inside Elections: toss-up
• Cook Political Report: toss-up
• Sabato's Crystal Ball: toss-up

Why is it in play?

Claire McCaskill is a notably centrist Democrat. She’s butted heads with more progressive members of her own party, including Kirsten Gillibrand (over Gillibrand’s military-sexual-assault bill) and recently came out against Medicare for All. She voted against only seven of Trump’s Cabinet appointments and almost voted to approve Supreme Court nominee Neil Gorsuch. McCaskill has plenty of practical reasons to edge away from the left: Trump won Missouri by 19 points, and her disapproval rating is hovering around 42 percent, making her the 14th-most unpopular senator. She’s a shrewd campaigner: She first won election to the Senate in an extremely competitive 2006 race, and in 2012, she helped the weakest Republican primary candidate, Congressman Todd Akin, win the party’s nomination by running $1.7 million in ads lambasting his paleoconservative views, a strategy that endeared him to the state’s most extreme voters and alienated him from everyone else. (She ended up spending more on Akin in the last two weeks of the primary than Akin spent on his entire campaign, and it worked.) This time, however, she might not get so lucky. McCaskill has spoken about her fear of a primary opponent, claiming back in February that “people are very impatient with me because they don’t think I’m pure.” Not every Democrat seems to share those views: Grassroots groups helped McCaskill bring in a record-breaking $2.8 million in the first quarter of the year.

Who are the challengers?

The Republican primary field is shaping up to be messier than expected, which could give McCaskill an advantage. U.S. representative Ann Wagner declined to enter the race, leaving 37-year-old state attorney general Josh Hawley as the front-runner. Though he’s clearly leading the field and has the backing of Mike Pence and Mitch McConnell, he’s only held office since January, and at 37, he’s the youngest attorney general in the country. Following reports that Steve Bannon might back a more conservative candidate in the race, as he did in Alabama, Hawley began courting the former White House adviser and talking up his conservative bona fides. Still, it’s possible that Bannon could lend his support to a more conservative candidate, like ex-Libertarian presidential candidate Austin Petersen, Air Force veteran Tony Monetti, or Courtland Sykes, a 37-year-old newcomer who’s trying to rely on social media and anti-Establishment rhetoric to gin up support for his campaign.

How vulnerable is the seat?

• Inside Elections: tilt democratic
• Cook Political Report: likely democratic
• Sabato's Crystal Ball: leans republican

Why is it in play?

This two-term senator has the kind of bona fides you can’t fake: Born and raised in Montana, he still maintains the farm that’s been in his family for more than 100 years, and lost three fingers in a meat saw as a child. When he last ran for reelection, he had an A rating from the NRA, and he voted against the DREAM Act. Still, Trump won the state by 20 points and Romney by 14. Opponents try to paint him as a Washington insider, and he’s been hit with ads criticizing his support of Obamacare and vote against Supreme Court nominee Neil Gorsuch. Tester has never had an easy election. He eked out his first win against a three-term incumbent in 2006, and in 2012, he only barely won reelection against Denny Rehberg, a six-term House representative. (Larry Sabato thought this was the Senate seat most likely to switch parties that year.) However, Tester’s approval rating has jumped to 53 percent since July, making him one of only two vulnerable senators to see an increase there.

Who are the challengers?

Republicans initially had difficulty recruiting candidates: U.S. representative Ryan Zinke was widely expected to challenge Tester until he was chosen by Trump to become the secretary of the Interior, and Montana attorney general Tim Fox decided against a bid back in June. Since then, at least five Republicans have declared their candidacy, with the most serious challenges coming from Montana state auditor Matt Rosendale and former Yellowstone County district judge Russell Fagg. Rosendale raised over $400,000 last fundraising quarter and is campaigning on a platform to improve Defense Department funding and strengthen support for veterans, who make up about 10 percent of Montana’s population. Though he hasn’t run for statewide office, Fagg has name recognition in Yellowstone County, which holds about a seventh of the state’s population and is key to Tester’s success. Local businessmen Troy Downing and Ronald Murray, and state senator Al Olszewski are also making bids for the seat.

How vulnerable is the seat?

• Inside Elections: toss-up
• Cook Political Report: lean democratic
• Sabato's Crystal Ball: leans democratic

Why is it in play?

During her only term in the Senate, Heidi Heitkamp has pushed for farm subsidies, an “all-of-the-above” energy policy, and Second Amendment rights. She was one of four Democrats who voted against new background-check legislation in the wake of the Newtown shooting, and supported Supreme Court nominee Neil Gorsuch. (So far this year, she’s voted in line with Trump’s position a little more than half the time.) Despite what seem to be fairly conservative bona fides and a 55 percent approval rating, Heitkamp’s reelection chances are dampened by Trump’s 36-point win in the state. She waited until September to even declare her intentions of seeking reelection.

Who are the challengers?

Republican state senator Tom Campbell is Heitkamp’s only declared challenger so far. Campbell, who makes his living as a farmer and has only served in the state senate since 2012, has criticized Heitkamp’s support of Obamacare and wants to balance the budget. He’s raised a little over half a million since entering the race in August; by comparison, Heitkamp’s raised a little over twice as much since July. Congressman Kevin Cramer, the state’s only U.S. representative, is seen as a potential challenger and met with Mitch McConnell back in December when Heitkamp was being considered for a Cabinet position, which would have left her Senate seat open. He has a history of controversial remarks, defending Sean Spicer after his “Holocaust centers” comments and mocking the white pantsuits women Democrats wore to Trump’s first address to a joint session of Congress. However, Republicans reportedly fear a Todd Akin–level gaffe and have largely lined up behind Campbell instead. State representative Rick Becker is also “seriously considering” a bid. So far no serious Democratic primary challenger has emerged.

How vulnerable is the seat?

• Inside Elections: toss-up
• Cook Political Report: toss-up
• Sabato's Crystal Ball: toss-up

Why is it in play?

With Jeff Flake sitting out 2018 in Arizona, Dean Heller is the Republican senator most vulnerable to a primary challenge from the right. The first-term senator’s troubles date back to the election, when he refused to endorse the Republican nominee. Under Trump, he’s drawn ire from both sides for his waffling; he voted against the Affordable Care Act, then publicly opposed the House repeal, even as tape surfaced of him saying he’ll“do everything I can to get to a yes.” At a rowdy town hall in April, Heller, who has a pro-life voting record, stated that he opposed defunding Planned Parenthood. Nevada’s blue streak also works against Heller: Senator Catherine Cortez Masto won Harry Reid’s former seat in 2016 with a majority in just one county, thanks in large part to the left-leaning cities of Las Vegas and Reno. Clinton and Obama won in the last three presidential elections, and in 2016, Democrats flipped two Republicans in the House and took both chambers in the state legislature.

Who are the challengers?

First, Heller must make it out of the primary. Danny Tarkanian, an attorney who’s been unsuccessful in his bids for state senate, the Senate, the House, and the Nevada secretary, finally has momentum, thanks to his unwavering support of the president and a cosign from Steve Bannon. The son of famed UNLV coach Jerry Tarkanian, the 55-year-old enjoys strong name recognition and a reputation for solid fundraising. In August, he led in the primary in at least one poll –– though a $17 million judgment over an unpaid loan and a sanction for practicing law with an inactive license could derail him in the long run. Heller’s threat from the left is Congresswoman Jacky Rosen, who defeated Tarkanian in the 2016 bid for the open seat in Nevada’s far-southern Third District. Already, Rosen has the blessing of retired Silver State powerhouse Harry Reid and $1.2 million in hand to fight the survivor of the Republican primary. She could make gains among the majority of Nevadans who voted for gun-control reform on the 2016 ballot, and which still haven’t been enacted after the Las Vegas shooting.

How vulnerable is the seat?

• Inside Elections: lean democratic
• Cook Political Report: lean democratic
• Sabato's Crystal Ball: leans democratic

Why is it in play?

A dovish, economic populist, Sherrod Brown represented a northern-Ohio district in the House for 14 years before riding a wave of anti-Iraq momentum into the Senate in 2006. Brown is something of a blue star in a bleak state for Democrats — the GOP has a super majority in both state house chambers, and Trump won Ohio by eight points. The senator has held on with a commitment to pro-union and pro-labor politics, including an endorsement for theFight for $15. Since November, he has been one of the leading Democrats to cooperate with the president, reaching out with a skeptical optimism on trade, jobs, and health care. Yet Brown maintains that the Cabinet has been handpicked for a“rollback of protections for workers”and recently said that Stephen Miller “seems to be” a white supremacist. In 2016, Trump won an estimated one-third of the votes cast by members of the United Auto Workers, the type of lunch-pail voters that Brown counts as part of his core constituency. After Trump’s blue-collar victory, Republican movers see 2018 as a banner year to remove Brown, whose 47 percent approval rating is just a point and change higher than the president’s rating in Ohio. Plus a 2018 loss would reduce Brown’s chance at his own populist presidential run in 2020.

Who are the challengers?

Ohio treasurer and Iraq War veteran Josh Mandel, who lost to Brown in 2012, is running again in 2018. Mandel, 40, has matched Brown’s economic populism with an appeal to nativism. In his announcement for 2018, Mandel promised“no sanctuary cities,” never to “succumb to political correctness,” and never to forget “we are a nation founded on Judeo-Christian values.” So far, Mandel has received the endorsement of the Cleveland police union (who also went for Trump) and substantial backing from the tea-party-affiliated FreedomWorks PAC, and Ted Cruz’s senior strategist Jeff Roe is on the team. In a poll this past July, Mandel was favored by Ohio voters by eight points, though Brown still holds the fundraising advantage, with twice as much cash on hand as Mandel and a strong $2.6 million third quarter. Mandel also faces a primary challenge from Cleveland businessman Mike Gibbons, who has called for the resignation of Mitch McConnell and is the favored candidate of the LLCCitizens for Trump.

How vulnerable is the seat?

• Inside Elections: lean democratic
• Cook Political Report: likely democratic
• Sabato's Crystal Ball: likely democratic

Why is it in play?

After Trump won Pennsylvania by 44,000 votes and incumbent Republican Pat Toomey held onto his seat in themost expensive U.S. Senate race ever, reelection prospects didn’t look great for the two-term incumbent Bob Casey. A centrist, pro-life Democrat, Casey had a quiet career in the capitol after knocking out the unpopular Rick Santorum in 2006; thePhiladelphia Daily News once called him “the political equivalent of Ambien.” But in recent months, the 57-year-old lawyer has awakened to the Trump resistance, calling Republican tax reform a “giveaway to the rich,” holding a press conference at a suburbanPlanned Parenthood center in Darby, Pennsylvania, promising to defend the institution’s funding, and calling out the president with reaction GIFs on Twitter. Casey enjoyed his own viral success when he rushed to the Philadelphia airport from a gala to negotiate for travel-ban detainees, still wearing his fundraising tux. If Pennsylvania’s$113 million Senate race in 2016 has set a precedent, a serious bid would mean serious money — already, Casey has $7 million on hand to fight for his third term on a platform of middle-class defense and “standing strong against terrorism.” Still, his pivot to the left has created space for a Republican to convert his centrist voting base. After the Las Vegas shooting, the historically pro-gun senator supported Dianne Feinstein’s bill to ban the sale of bump stocks, and he appears to be hedging his stalwart pro-life stance.

Who are the challengers?

Four-term congressman Lou Barletta is the most prominent Republican candidate to declare. An early supporter of Trump, Barletta may try to replicate the coal-and-populism angle that won the presidential race — he’s an extreme border-security hard-liner and supports “clean-coal technology,”as well as further exploration for natural gas in the Marcellus Shale. Barletta is second among Pennsylvania GOP candidates in fundraising, almost $300,000 shy of the $1,024,930raised by businessman Jeff Bartos, who is running on a similar platform with the addition of senatorial term limits.

How vulnerable is the seat?

• Inside Elections: solid republican
• Cook Political Report: likely republican
• Sabato's Crystal Ball: likely republican

Why is it in play?

In September, Bob Corker, a second-term senator and head of the Committee on Foreign Relations, became the first senator to announce his retirement. He followed that up with a series of candid interviews about Trump, and an infamous tweet comparing the White House to an adult-day-care center. Trump’s disapproval rating in the state has also gone up 14 points since January, though he won it by 26 points last fall. Still, Tennessee hasn’t sent a Democrat to the Senate since 1990.

Who are the challengers?

Leading the Republican side is U.S. representative Marsha Blackburn, who’s represented the state since 2003 and has closely aligned herself with Trump. In a video announcing her candidacy, she called herself “politically incorrect and proud of it.” She’s also filed a resolution requiring NFL players to stand during the anthem and voiced support for Trump’s immigration ban. Former Republican representative Stephen Fincher is also making a bid, as is Andy Ogles, former head of the Tennessee chapter of the Koch-funded group Americans for Prosperity. Democrat James Mackler, a Nashville lawyer and Iraq War veteran, announced his long-shot bid against Corker back in April. His chances against Blackburn are a dramatic improvement, allowing him to act as the rational center to her extreme-right views. However, she also has $3.2 million on hand, compared to Mackler’s $320,000. Since Corker’s announcement, former Democratic governor Phil Bredesen has reportedly begun considering a bid. Bredesen won election by a landslide in 2006 (the last Democrat to win statewide office), and was known for his fiscally conservative, centrist politics. Many Democrats consider him key to winning the seat. A late October poll found that Blackburn and Bredesen had “statistically indistinguishable” approval ratings, at 37 percent and 34 percent, respectively. But whoever wins the nod will make a better challenger than 2012 candidate Mark Clayton, a conspiracy theorist who was disavowed by the Tennessee Democratic Party for his connection to an anti-LGBT hate group.

How vulnerable is the seat?

• Inside Elections: tilt democratic
• Cook Political Report: likely democratic
• Sabato's Crystal Ball: leans democratic

Why is it in play?

After 14 years representing Wisconsin in the House, Baldwin became the first openly gay person elected to the Senate when she won in 2012. As a one-term senator with a very progressive voting history, she’s a target for Republicans hoping to capitalize on Trump’s surprise upset in the state last fall. Senate races in Wisconsin have also traditionally received a lot of outside spending, and this race is likely to be very expensive. The Koch Brothers–affiliated group Freedom Partners has already dropped $1.6 million on TV ads slamming Baldwin’s 2011 vote to raise taxes. The Senate Majority PAC fired back with a similarly hefty ad buy touting Baldwin’s record of cutting taxes for the middle class. Outside groups have already dumped more than $5 million into the race, most of which has gone toward either opposing Baldwin or supporting her main opponent. Baldwin also has a 38 percent disapproval rating, one of the highest among vulnerable senators (only Claire McCaskill and Dean Heller have higher disapproval ratings, with 39 percent each).

Who are the challengers?

Kevin Nicholson, a 39-year-old Marine veteran and former McKinsey consultant, has won over influential Republicans in the state. And while his campaign has only raised a little over $400,000 since entering the race in July, the conservative megadonors Richard and Elizabeth Uihlein has given $3.5 million to a pro-Nicholson PAC. Nicholson was involved in Democratic politics when he was younger and was the president of the College Democrats of America. As part of the gig he got to give a short speech at the 2000 convention, in which he spoke about “generational differences” and emphasized “a woman’s right to choose.” State senator Leah Vukmir also announced a bid in September and has raised an impressive quarter-million since then. Nicholson’s Democratic past has given Vukmir an opening, and she’s emphasized her working relationship with Governor Scott Walker and long history with conservatives in the state (she’s already won support from several key Wisconsin conservatives). Vukmir, a nurse, has criticized Baldwin’s support for Medicare for All, but it’s unclear how much that will resonate in a state that Bernie won by more than 13 points in the primary.

How vulnerable is the seat?

• Inside Elections: toss-up
• Cook Political Report: toss-up
• Sabato's Crystal Ball: leans democratic

Why is it in play?

Two-term senator Joe Manchin has the most conservative voting record of any Democratic senator and is frequently criticized for being a “Democrat in name only.” He had the fewest Cabinet objections of any Senate Democrat, voting against only four nominees (DeVos, Mulvaney, Price, and Ross). He is a tax-slashing EPA opponent who’s also widely respected by Republicans both in Washington and his home state. In 2012 Manchin received an NRA endorsement but also pushed for stronger gun-control legislation after the Newtown shooting. He seems closer to the president than even some Congressional Republicans — the two call each other on their cell phones, and Manchin was briefly considered for a Cabinet position. He’s voted in line with Trump’s position more than half of the time. Although he’s not as vilified by Republicans as others on the left, and maintains 53 percent approval among his constituents, Manchin’s still a clear target in a state Trump won by by 42 points (West Virginia hasn’t voted for a Democratic president since 1996, and in 2014 Republicans flipped the state legislature for the first time in 83 years). Trump is also still extremely popular in the state, with a nearly 60 percent approval rating.

Who are the challengers?

The two Republican front-runners, state attorney general Patrick Morrisey and U.S. representative Evan Jenkins, are already locked in a contentious battle for the nomination. Both are trying to prove their close ties to Trump and commitment to West Virginia’s coal-focused economy. Jenkins has released a TV ad claiming Morrisey used to be a #NeverTrump-er, while Morrisey has capitalized on Jenkins’s past as a Democrat (he switched parties in 2013). Morrisey has also tried to emphasize his past work fighting against Obama-era environmental regulations, while Jenkins has attacked Manchin’s gun-control record, specifically his vote for increased background checks in the wake of Newtown. Political newcomer Bo Copley could possibly benefit from a vicious race between Jenkins and Morrisey. A laid-off coal miner, Copley gained attention during a 2016 town hall with Hillary Clinton in which he got her to apologize for anti-coal statements she had made, and he’s been trying to tie Manchin to Clinton. However, he’s never held public office and his platform of moving beyond coal jobs sounds strikingly similar to that of Democrat Paula Jean Swearengin, also a coal-miner’s daughter and political novice who so far is Machin’s only primary challenger. Swearengin is unlikely to gain much headway, but could open up Manchin to criticism for being unpopular even within his own party.