But the reality is that Republican lawmakers have been put in this position because Mr. Trump was more popular among Republicans than anyone else in the race. He took the lead in the national polls in July 2015 and pretty much never lost it. Those of us who oppose Mr. Trump need to acknowledge that his European-style ethnic nationalism, which relies on stoking grievances, resentments and fear of the other — Mexicans, Muslims, Syrian refugees, the Chinese, etc.— has a powerful sway in the Republican Party today. To be clear, not all of Mr. Trump’s supporters are drawn to his ethnic nationalism. But all of his supporters are willing to accept it.

This is not the conservatism of William F. Buckley Jr. or Ronald Reagan or Jack Kemp; it is blood-and-soil conservatism primarily aimed at alienated white voters who believe they have lost the country they once knew. Trumpism also includes a heavy dose of conspiracy theories. It is no coincidence that Mr. Trump burst onto the national political scene in 2011 by claiming that Barack Obama, our first black president, was not a natural-born American citizen but rather was born in Kenya.

Mr. Trump knows his target audience, which explains why, beginning the morning of the Indiana primary on May 3 (the day he became the de facto nominee), he has — among other in-the-gutter moments — implied that Senator Ted Cruz’s father was implicated in the assassination of President Kennedy; insinuated that Vince Foster, a friend of the Clintons who was White House deputy counsel, was murdered (five official investigations determined that Mr. Foster committed suicide); engaged in a racially tinged attack on Gonzalo Curiel, a district court judge presiding over a fraud lawsuit against Trump University; and expressed doubt that a Muslim judge could remain neutral in the case. This is conspiratorial craziness and rank racism — and all of it has happened after we were told Mr. Trump would raise his game.

The surprise is that so many Republicans are now expressing consternation at what Mr. Trump is doing. Has any recent presidential candidate ever advertised quite as openly as Mr. Trump the kind of vicious attacks he’d engage in? We were warned in neon lights what was coming. The idea that he will now engage in a “course correction” — that he will flip a switch and transform himself into a decent and dignified man — is laughable. Mr. Trump has repeatedly stated that he won’t change his approach. (“You win the pennant and now you’re in the World Series — you gonna change?”) In this one area, Republicans should take him at his word.

When a narcissist like Mr. Trump is victorious, as he was in the Republican primary, and when he has done it on his terms, he’s not going to listen to outside counsel from people who think they can change the patterns of a lifetime. Republicans have not changed Mr. Trump for the better; he has changed them for the worse.

So here we are, with Republicans who lined up behind Mr. Trump now afraid of being led off a high cliff. If the prospect of a November shellacking isn’t enough to unnerve these Republicans, there’s also this to factor in: What we are talking about is potential generational damage to the Republican Party.

Consider this historical comparison: In 1956 the Republican nominee, Dwight D. Eisenhower, won nearly 40 percent of the black vote. In 1960, Richard Nixon won nearly a third. Yet in 1964, in large part because of his opposition to the Civil Rights Act, Barry Goldwater (who was no racist) won only 6 percent. More than a half-century later, that figure has remained low. Mr. Trump — through his attacks against Hispanics that began the day he announced his candidacy — is doing with Hispanics today what Senator Goldwater did with black voters in the early 1960s.