President Donald Trump's Senate impeachment trial promises to leave him more powerful in Washington — and possibly more vulnerable to defeat on the campaign trail.

That's in part because a handful of pivotal Senate Republicans chose to criticize Trump's behavior in office while protecting him from both official sanction and the potential jeopardy of witnesses unraveling his impeachment defense under oath. As a result, Trump is on the verge of emerging from the trial with a tacit green light to defy Congress without fear of reprisal, and also safe in the knowledge that elected representatives will push only so far to find out whether he tells the truth to the public.

"It’s arguable that he’s the most politically powerful president in American history," presidential biographer Jon Meacham said on NBC News during a break in the trial Friday.

But that power, demonstrated with the Senate's 51-49 vote Friday against considering new evidence, combined with the mild rebukes from GOP senators to dilute the most compelling aspect of his political brand. It will be harder for Trump to cast himself as a victim of the system after allies in the Senate said he overstepped the bounds of his authority and then used their power to bail him out of trouble.

The more he looks like he's rigging the system, the less it looks rigged against him.

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When President Richard Nixon resigned in August 1974, his successor, Gerald Ford, told the nation that “our long national nightmare is over.” But with Alan Dershowitz’s arguments during President Donald Trump's impeachment trial last Wednesday that a president can do almost anything “that he believes will help him get elected — in the public interest,” it is clear that Nixon’s resignation left a serious gap in the precedents of impeachments.

Indeed, Dershowitz may have some of the last words on the matter. On Friday, the Senate voted to not allow new witnesses, including John Bolton. It seems increasingly likely that the Senate will vote soon to acquit Trump. So what went wrong here, if you believed conviction was appropriate? The answer starts with the Nixon precedent, or better said, the lack of precedent.

The precedents set by each impeachment are important. And what happened to Nixon can help explain what happened, however different, to Trump.

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Here's a brief look at what happened this week in Trump's impeachment trial:

• GOP Sen. Susan Collins of Maine announces she will vote to acquit Trump
• Schiff's powerful closing speech: 'Is there one among you who will say, Enough!'?
• Alaska Sen. Lisa Murkowski, a moderate Republican, says Trump's actions 'shameful' but she'll vote to acquit
• West Virginia Sen. Joe Manchin, a moderate Democrat, introduced a resolution to censure the president instead of removing him from office.