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…

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.

Might a President Trump show the same flexibility and appetite for pragmatic maneuver? One hopeful sign is something that alarms conservatives: Mr. Trump’s friendly past relations with Democrats. A second is his indifference to hot-button social and cultural issues like transgender bathrooms.