This is the cycle Mr. Trump’s nomination could potentially break. And this threat largely explains the antipathy toward him coming from many establishment Republicans and their intellectual allies. To be fair, some conservatives have objected to genuinely troubling aspects of Mr. Trump’s character and his campaign — in particular, his bigoted remarks about immigrants, Mexicans and Muslims.

These are serious cause for concern, but not, historically, for conservatives, who for many years have exploited racial and ethnic divisions for political gain. Goldwater’s own national campaigns were built on courting Southern segregationists, which he began to do as early as 1961. Ronald Reagan crudely caricatured welfare recipients. Ted Cruz has called for patrolling “Muslim neighborhoods.”

The actual conservative brief against Mr. Trump reflects something else — his rejection, or ignorance, of ideological boilerplate. “He basically never says ‘freedom’ or ‘liberty.’ He gives no indication of caring about the Constitution,” the editor of National Review, Rich Lowry, and a senior editor at the magazine, Ramesh Ponnuru, wrote last October, when it was becoming clear that Mr. Trump was making a strong presidential run. “He has, in short, ignored central and longstanding conservative tenets that seemed to have become only more important in the Tea Party era.”

True enough. But those certitudes don’t begin to address the concerns of the large base of voters who have yet to feel the benefits of the economic recovery. When Mr. Cruz called Mr. Trump “a big-government liberal, just like Barack Obama and just like Hillary Clinton,” he actually touched on a prime cause of Mr. Trump’s wide appeal. He has accurately read the mood of restive voters. The loudest anti-government rhetoric often muffles the urgent cry for more, not less, government. As early as 2010, surveys showed that Tea Party supporters who said they favored smaller government were loath to give up giant programs like Social Security and Medicare.

Recently Mr. Trump has been tacking to the right. His early promise to curtail tax breaks for hedge-fund billionaires (“getting away with murder”) seems to have been smudged over in a tax plan that resembles Mr. Ryan’s supply-side blueprint. But he also presents himself as a negotiator and compromiser who will unlock the frozen gears of government. This has led old-style Republican legislators like Bob Dole and Trent Lott to say they preferred him to the harder-edged Mr. Cruz, with his checklist of “principles.”

The challenge for Mr. Trump will be convincing voters that he really does mean to improve conditions for working-class and middle-class Americans and not just play to their grievances. If he does, and brings the party along with him, he could be a formidable foe to Mrs. Clinton, especially if the email controversy continues to dog her. But even in crushing defeat, he could be a kind of reverse Goldwater who shifts the party closer to the center. Trumpism, if not Mr. Trump himself, might return the party to the pragmatic conservatism of presidents like Eisenhower and Nixon.

Indeed, Nixon could provide the best template. Like Mr. Trump, he ran a polarizing campaign. Its “law and order” message appealed to blue-collar whites who felt menaced on all sides — by civil-rights demonstrators, anti-Vietnam War protesters and liberal intellectuals who seemed disdainful of traditional American values. But Nixon was not an ideologue. He recruited Daniel Patrick Moynihan, a Democrat, to rethink urban policy. Nixon also guided the nation out of Vietnam, normalized relations with the Soviet Union and opened talks with China.