Will impeachment change Trump? A Washington guessing game.

For the first time in American history, an impeached president is running for reelection – and quite possibly could win. That stark reality came to life Tuesday night, as President Donald Trump delivered his third State of the Union address, a more raucous campaign-style event dotted with made-for-TV moments than the often-staid annual affair. 

The president delivered last night’s speech amid the expectation of acquittal today by the Senate. A midafternoon announcement from Utah Republican Sen. Mitt Romney that he would vote to convict Mr. Trump on one count dealt a blow to the Republican wall of unity. But the president remained a leader on the verge of triumph, with his Gallup job approval rating at an all-time high (49%) and his political adversaries warning of a chief executive unbound. 

The icing on the cake this week came from Iowa, where the fiasco over the Democrats’ inability to report the results of Monday’s presidential caucuses in a timely fashion handed Mr. Trump another political gift. 

Still, questions abound: Will the president overplay his hand? Post-acquittal, will he feel unfettered in his behavior as he seeks a second term? While some Republican senators have said that his dealings with the Ukrainian president – withholding military aid in exchange for dirt on political opponents – were “shameful,” all but Senator Romney voted today to acquit.

Senator Romney’s stunning decision to vote to convict on one count – abuse of power – no doubt took some of the wind out of Mr. Trump’s sails. Calling himself “profoundly religious,” the former GOP presidential nominee said he couldn’t think of much “that would be a more egregious assault on our Constitution than trying to corrupt an election to maintain power. And that’s what the president did.”

The logical approach for Mr. Trump, political analysts say, would be to restrain his behavior and not do anything too risky through Election Day. After all, he can claim vindication on the impeachment charges; the economy is strong; a near-record-high 59% of Americans say their personal finances are better now than a year ago, according to Gallup; and Democrats are in disarray. As things look now, Mr. Trump is in a reasonably good position to win in November. 

“It seems like the people around him would say, ‘Just run the string here, then you’ll have four more years and you can do what you want to do,’” says Cal Jillson, a presidential scholar at Southern Methodist University in Dallas. 

Aides had reportedly advised the president not to mention impeachment in the State of the Union, following President Bill Clinton’s 1999 precedent, and the fact that the president stuck to that advice Tuesday night was noteworthy – suggesting he may be willing to exercise a bit more discipline three years into his presidency. Mr. Trump, after all, is famous for over-the-top rhetoric, via Twitter and at rallies, and his cries of “witch hunt” (or similar charges) have been part of his repertoire since he announced his candidacy in 2015. 

Yet the cloud of impeachment hung over the proceedings, and its accompanying partisan rancor contributed to a circuslike atmosphere. House Speaker Nancy Pelosi and Mr. Trump publicly snubbed each other at the beginning and end – most dramatically, when Speaker Pelosi ripped a paper copy of his speech in half after he finished. Some Democrats heckled and gestured over a pronouncement on health care; some walked out. 

Moderate Sen. Susan Collins of Maine, one of only two GOP senators (along with Senator Romney) to vote in favor of witnesses during the Senate trial, argued Sunday that Mr. Trump has learned his lesson. 

“He was impeached,” Senator Collins said on CBS’s “Face the Nation.” “And there has been criticism by both Republican and Democratic senators of his call [with the Ukrainian president]. I believe that he will be much more cautious in the future.” (Though she later said that “hope” may have been a better word than “believe.”)

One big difference between the Clinton and Trump impeachments is that Mr. Clinton’s took place toward the end of his presidency, and Mr. Trump’s is taking place amid a reelection bid. 

But there’s also a big difference in their respective postures. With Mr. Clinton, “there was this general sense that he was chastened and remorseful,” says Russell Riley, co-chair of the Presidential Oral History Program at the University of Virginia’s Miller Center. “With Trump, there’s none of that. So there’s an enormous amount of anxiety about what he might do as president from this point forward, because he’s effectively a president unbound.” 

Professor Riley sees this period as constitutionally perilous.

In the past, “institutions balanced other institutions, but those checks and balances are largely not working,” he says. “So the only real question is the sense of self-restraint that President Trump feels as it relates to his own reelection prospects.” 

Trump defense lawyer Alan Dershowitz stoked fears last week that the president’s executive power could become unfettered, when the retired Harvard Law professor asserted, “If a president does something which he believes will help him get elected in the public interest, that cannot be the kind of quid pro quo that results in impeachment.” 

Mr. Dershowitz later said his remarks were mischaracterized, but the larger concern about presidential overreach remains. 

House Democrats, meanwhile, say more information will emerge about the president’s actions in Ukraine. One possibility could be testimony in House hearings from John Bolton, a former Trump national security adviser, and Lev Parnas, the Ukrainian-born former associate of Trump personal lawyer Rudy Giuliani. 

In an interview Sunday on “Face the Nation,” House Intelligence Chairman Adam Schiff would not state whether Democrats plan to subpoena Mr. Bolton. But, he added, “the truth will come out” – whether in testimony before the House or in Mr. Bolton’s forthcoming book.