Earlier today, hours before Trump's rally, Biden tweeted about his support for gun control, noting Trump's opposition.

House Majority Leader Steve Scalise, R-La., decried Democrats' campaign-season rhetoric warning that a second Trump term would curb democracy, saying it fuels political violence.

"For weeks Democrat leaders have been fueling ludicrous hysteria that Donald Trump winning re-election would be the end of democracy in America," the congressman said on social media platform X after the incident at Trump's rally.

"Clearly we’ve seen far left lunatics act on violent rhetoric in the past," he said.

Scalise was one of four people wounded when a gunman opened fire on members of the Republican congressional baseball team practicing in Alexandria, Virginia, in June 2017.

NBC News' Dasha Burns spoke to a witness at the scene in Butler who said she was in the front row, directly in front of Trump, when shots appeared to ring out.

The woman, who identified herself only as Erin, said she heard four pops three to four minutes into the rally. Secret Service immediately jumped on the stage, she recalled.

When Trump got up, she said she saw blood on Trump’s ear. “No one was concerned about us. We were only concerned about him,” Erin said, referring to Trump.

She also said she saw someone covered in blood who appeared to be next to someone who was “bleeding profusely” and had seemingly been shot.

Erin said a man sitting next to her in the crowd said he saw a shooter on a nearby water tower.

"If you don't like the guy, don't vote for him," she said, again referring to the former president. "Don't kill him. ... What's with the violence?"

Former President Barack Obama said in a statement that there's no place for political violence in the U.S. and everyone should be relieved that Trump is reported to be OK.

“There is absolutely no place for political violence in our democracy. Although we don’t yet know exactly what happened, we should all be relieved that former President Trump wasn’t seriously hurt, and use this moment to recommit ourselves to civility and respect in our politics," Obama said. "Michelle and I are wishing him a quick recovery.”

Wisconsin Gov. Tony Evers, a Democrat, said his “thoughts are with President Trump and all of the folks attending today’s rally in Pennsylvania," and he and his staff are in touch with the RNC about security at next week's convention in Milwaukee.

"My staff and I are in contact with those coordinating security planning for the RNC and will continue to be in close communication as we learn more about this situation," Evers said in a statement on X. "We cannot be a country that accepts political violence of any kind—that is not who we are as Americans."

The shooter from the Trump rally is dead, Butler County's district attorney says.

One spectator is also dead and another is in "serious condition," the DA says.

Two witnesses told NBC News’ Dasha Burns that they saw two people down. The witnesses’ accounts are unconfirmed.

The witnesses, a mother and son, say this makes them hope Trump wins “more than ever” to “send a message to these people.”

“They can’t stop us,” one of the witnesses said.

Vice President Kamala Harris has received an initial briefing on the incident at Trump’s rally, the Office of the Vice President said.

Violent threats directed at public officials have been growing sharply in recent years, according to data from West Point's Combating Terrorism Center.

"While 2013-2016 had an average of 38 federal charges per year, that number sharply increased to an average of 62 charges per year between 2017-2022," according to a West Point report published in May. "Across the time series, ideologically motivated threats, on average, accounted for almost half of the cases, and the portion steadily increased year over year. A preliminary review of cases from 2023 and 2024 shows that the number of federal prosecutions is on pace to hit new record highs."