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.