[We want to hear from you. Are you going to watch the Democratic debates?]

At least some Republicans argue that Mr. Trump represents the delayed recalibration of a party that defined itself for decades by its opposition to the Soviet Union. Without that unifying belief system, the party has begun to drift back to some of its historic roots before World War II, the last time the phrase “America First” was popular.

“He’s really broken the mold in a lot of ways, and he’s really moved the Republican Party away from the traditional Reagan stool,” said Antonia Ferrier, a Republican consultant who worked for Senator Mitch McConnell of Kentucky and other party leaders on Capitol Hill. “But whether it’s him or it’s history, the reality is the post-Cold War paradigm for the Republican Party was going to be changed one way or the other.”

Richard Norton Smith, the historian and former director of presidential libraries of five Republican chief executives, suggested there could be a parallel with Ronald Reagan’s election in 1980, which, he noted, was “widely viewed as a one-time reaction against the Carter years” while “1984 was to demonstrate that the 40th president was far more transformational a figure than he appeared to his critics.”

At the same time, Mr. Smith hastened to distinguish “Reagan’s politics of multiplication from Trumpian division.” The parallel, he said, is not between the men but between re-election campaigns that may serve as a referendum on profound, even radical changes in national priorities.

Mr. Trump’s re-election campaign may also reveal more about how much the nation’s politics are driven by policy or personality. The support for and opposition to Mr. Trump often seems tied less to his specific policy prescriptions than to evaluations of who he is — a fighter taking on elites on behalf of those left behind or a vulgarian narcissist with no respect for the rule of law.

“I can’t imagine this is anything but a conversation about his persona,” Mr. Buck said. “To this day, everyone plays on his home turf. He defines what we talk about, he defines everything.”

Indeed, America is so divided into camps that few have moved since Mr. Trump’s election, and it remains unclear whether there is a sizable share of voters in the middle who are actually open to persuasion.

“His numbers have been remarkably stable for how unstable this presidency is, because it is so much more than do you support him,” Mr. Buck said. “There is a palpable belief among a lot of people — and it crosses both ways — that the other side hates you and is out to get you, and that keeps you firmly in whatever camp you’re in.”