Four Years Ago Trump Was Seen as a Sideshow. Now He Is the Show.

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.