Indeed he has, but not the lessons that Ms. Collins seemed to think. Listen to Rudy Giuliani, the president’s personal lawyer and Ukraine whisperer. “Absolutely; 100 percent,” Mr. Giuliani told NPR when asked whether Mr. Trump would continue to push for more foreign “investigations” into Joe Biden, a Democratic front-runner. “I would have no problem with him doing it. In fact, I’d have a problem with him not doing it.”

Even before the acquittal, the State of the Union address made clear that Mr. Trump — enabled, as in his business life, by his exceptional shamelessness — intends to deploy every power available to a president in pursuit of his re-election. If there remained any doubts on that score, they were dispelled when Melania Trump hung the Presidential Medal of Freedom around Rush Limbaugh’s neck.

The speech also demonstrated that Mr. Trump, unlike the Democratic Party, has a simple, powerful message: In three short years, he has brought America back from the disaster he claimed it was in and set it on a path to a glorious new future. From the “American carnage” he spoke of in his Inaugural Address, now, “America’s future is blazing bright.”

Given the dishonesty, if not downright absurdity, of some passages in the speech, it was perhaps a human reaction on the part of the House speaker, Nancy Pelosi, to tear it to shreds. It was still disappointing to see her stoop to the kind of stunt the president himself would pull, and, as she certainly knows as well as anyone, a gesture like that won’t defeat the president’s argument. So what will?

Not the incoherent, if not to say chaotic, display the Democratic Party has mustered to date. A series of overcrowded debates has failed to clarify the choices confronting voters. The decision to stage the first vote in monochromatic Iowa was a bad idea even before the Iowa Democratic Party had a meltdown on live television. A recent poll found that 45 percent of the supporters of the leading progressive candidate, Senator Bernie Sanders, said they weren’t sure they’d vote for any other Democratic nominee. Meanwhile, over in the party’s centrist camp, Michael Bloomberg appears to be trying to buy the nomination.

Given the stakes, anxiety is rightly running high. Yes, the Democratic voters can and should narrow the field over the next several weeks. But the fact that they’ll eventually pick a nominee isn’t enough. It matters how they get there. The 2016 election is a cautionary tale — too many Democrats felt so little allegiance to the nominee that they chose to vote for a third-party candidate, or not to vote at all. The task ahead for party leaders is ensuring that voters appreciate that the best chance to beat Mr. Trump is a unity of effort from political tribes that share far more in common than what divides them.

That’s important, because the November election is a critical opportunity to defend the Republic through the straightforward expedient of voting Mr. Trump from office and thereby issuing an essential rebuke to the leaders of today’s Republican Party. Republican officials in Washington and across the country have for years refused to acknowledge the legitimacy of political opponents, and have thus justified taking any and all measures to keep them out of power, or to nullify their power when they hold it.