WASHINGTON — As he rode down that escalator in June 2015, it felt like a lark, a curiosity, just another staged television spectacle. At most, many assumed that Donald J. Trump’s candidacy would be a sideshow, sure to be entertaining but hardly decisive.

Four years later, as President Trump kicks off his campaign for a second term on Tuesday with an eardrum-pounding, packed-to-the-rafters rally in Florida, no one doubts that he is the dominant force in the arena today, the one defining the national conversation as no president has done in generations.

But the coming election is shaping up as a test — not just of the man but of his country. Was Mr. Trump’s victory the last time around a historical fluke or a genuine reflection of America in the modern age? Will the populist surge that lifted him to the White House run its course or will it further transform a nation and its capital in ways that will outlast his presidency? What kind of country do Americans really want at this point?

Whatever voters thought about Mr. Trump in 2016, they have now had more than enough time to take their measure of him, and their judgment arguably will say more about the mood of the world’s last superpower than whatever roll-the-dice decision may have been made last time. Mr. Trump promised to blow up the system; voters will decide if more disruption is still needed.

“You go from what was probably a pretty low-stakes election to a high-stakes one,” said Brendan Buck, a former counselor to Paul D. Ryan, the last Republican speaker of the House. “When there were so many people who didn’t think he was going to win, who didn’t think it was even possible, your vote didn’t seem as important. That today has totally shifted.”

For Mr. Trump himself, the next year and a half will be a chance to prove that he was not just an aberration who managed to slide into office with an Electoral College victory even though nearly three million more voters cast ballots for the other candidate. Ever since taking the oath, Mr. Trump has been sensitive to perceived attacks on his legitimacy, especially the investigations into Russia’s role in helping to elect him. Nothing would do more to validate that legitimacy than winning a second term.

He starts from a stronger position in many ways than that day in 2015, fully in command of the advantages of incumbency — the gushing fund-raising spigot, the unparalleled media bullhorn, the tools of government to reward or punish, the big plane with “United States of America” stenciled on its side conveying power and respect.

And he has firmly seized control of his party in a way that was almost unimaginable in 2016 when Republican elders plotted ways to take the nomination away from him at the convention, then later urged him to drop out just weeks before the November election on the assumption that he would crash and burn.

The Never Trumpers have since faded, the dissenters purged. While elected Republicans push back from time to time, they have largely fallen in line when it really counted. The only challenger to Mr. Trump for his party’s nomination, former Gov. William F. Weld of Massachusetts, poses no evident threat.

“Trump is president because ‘the deplorables’ rejected the decline of America sanctioned by the elites,” said Stephen K. Bannon, the president’s former chief strategist. “The established order from Day 1 has fought a rear-guard action to nullify his election. This campaign is a continuation of the first — in all its vitriol and promise.”

Yet Mr. Trump remains more vulnerable than many presidents heading into a re-election year. Even with a strong economy, he is the only president in the history of polling who has never once, not for a single day, earned the support of a majority of Americans surveyed by Gallup. His own internal polls this spring showed him losing badly in key states, prompting him to first deny their existence and later to fire some of his pollsters.

Mr. Trump has never expanded his support beyond the people who elected him — and never really tried. He has remained focused intently on retaining the support of his base to the exclusion of reaching out to those who have opposed him. Whether by inclination or calculation, it is a strategy for a divided era when Americans are less interested in getting along.

There is, of course, something of a chicken-or-the-egg quality to the debate over Mr. Trump — is he the cause of America’s polarization or the result? The election may provide more clues.

“We’re very divided,” said Michael Kazin, a history professor at Georgetown University and co-editor of Dissent magazine, a left-of-center intellectual journal, “and we’ll learn whether the division is more about Trump or about something deeper in our ideological politics.”

Democrats have sought to treat Mr. Trump as if he were an outlier in the American story, one that can be corrected in the next election. “History will treat this administration’s time as an aberration,” former Vice President Joseph R. Biden Jr. has predicted on the campaign trail. The nation “can overcome four years of this presidency,” Mr. Biden has said, while a two-term Trump administration would pose an “existential threat” to “the character of this nation.”

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.”

Follow Peter Baker on Twitter: @peterbakernyt.

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