For the next three days, Rubio’s crude schoolyard taunts continued: over Trump’s made-in-China suits, his “horrible spray tan” and his small hands — a quip that, as Trump would later explain on the debate stage in Detroit, implied something else was small, too.

Rubio’s benefactors were aghast to see their candidate practicing Trump’s gutter politics. “Everyone went, ‘What?. . . Why are you going down to that level?’ ” recalled one fundraiser, who spoke on the condition of anonymity to talk candidly. “You might as well support Trump.”

Looking back, Rubio’s supporters see these fateful days as central to his unraveling. A strategy designed to get under Trump’s skin and force him on the defensive instead backfired on Rubio, diminishing the 44-year-old senator who had spent years trying to demonstrate presidential gravitas. At rally after rally, Rubio was unintentionally personifying the caricature that Trump was perpetually drawing of him: “Little Marco.”