Republican presidential candidate Donald Trump speaks in New York on May 3. (Jewel Samad/Agence France-Presse via Getty Images)

NOW THAT a reality-television star has all but secured their nomination, Republicans are scrambling to adjust — or hoping that Donald Trump will change.

Not surprisingly, GOP Chairman Reince (“Winning is the antidote to a lot of things”) Priebus was one of the first to hail the “presumptive @GOP nominee” Tuesday night on Twitter and call for party unity. Former Louisiana governor Bobby Jindal — who last fall recognized Mr. Trump as a “ narcissist,” “an egomaniac,” “a carnival act” who “believes in nothing” and is “absurd,” “insecure and weak” and “dangerous” — now says he will vote for the man. The Wall Street Journal editorial page, previously eloquent in its opposition, urges Mr. Trump to behave “in a way that reduces his epic unfavorable numbers with many voters.”

What would such a reboot look like? It could start with some relatively easy steps. Mr. Trump, like every modern presidential nominee before him, would release copies of his tax returns, which he has refused to do. Because he has no record in public office, he would also release sufficient documentation from his business affairs to allow voters to judge his accomplishments.

Then Mr. Trump would explain his policy proposals, which thus far have been mathematically impossible, absurdly vague, totally contradictory or some combination of the three. He promised, for example, to save $300 billion per year by allowing Medicare to negotiate with pharmaceutical companies over drug prices — when Medicare’s total annual spending on drugs is $78 billion. His tax cuts would reduce revenue by $1 trillion per year, even if you accept that they would generate economic growth. That means, as our columnist Ruth Marcus explained months ago, that he would have to cut one-fifth of all spending — and that’s before getting to his promise of paying off the entire federal debt in eight years. Which fifth would he cut? And how would he “quickly” destroy the Islamic State and prevent its resurgence, all without enmeshing the United States in foreign conflicts?

From policy, Mr. Trump might move to recanting the many lies he has told on his way to the nomination: that he saw thousands of Arab Americans in New Jersey celebrating the destruction of the World Trade Center on 9/11. That the Mexican government is sending rapists and other criminals into the United States. That the George W. Bush White House tried to silence him because he opposed the Iraq War. And so on.

Republican presidential front-runner Donald Trump visited the editorial board of The Washington Post on Mar. 21. Here is audio of the full, unedited interview. (Gillian Brockell/The Washington Post)

Finally, of course, Mr. Trump would have to atone for the hatred, bigotry, meanness and contempt for constitutional values that formed the foundation of his campaign: the mocking of a disabled reporter. The proposals to ban Muslims and round up Hispanics. The glorification of violence against protesters. The nasty threats to political opponents. The casual stereotyping of Jews. The winking at endorsements from white supremacists. The promise to torture prisoners and kill the innocent relatives of suspected terrorists. And so on.

No such reboot is conceivable, of course. It is possible that Mr. Trump henceforth will appear more “presidential,” if by that is meant speaking in a calmer tone and spewing fewer insults — though his National Enquirer scandal-mongering on the day of his great victory is not an encouraging portent. But he cannot change who he is or how he seized this moment.

His nomination is a calamity for the Republican Party, and Republicans will have to sort out how it happened and how they might recover. But the more urgent task is to ensure that Mr. Trump does not become a calamity visited on the United States and the world. For all his unpredicted success, the number of Americans who have voted for him so far amounts to only 4.7 percent of eligible voters, according to a calculation by the organization FairVote. As conservatives of principle recognize, he can be stopped, and he must be.