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 Trump's candidacy would be a sideshow, sure to be entertaining but hardly decisive.

Four years later, as Trump kicks off his campaign for a second term Wednesday 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