Monday’s presidential debate was a blowout, surely the most one-sided confrontation in American political history. Hillary Clinton was knowledgeable, unflappable and — dare we say it? — likable. Donald Trump was ignorant, thin-skinned and boorish.

Yet on the eve of the debate, polls showed a close race. How was that possible?

After all, the candidates we saw Monday night were the same people they’ve been all along. Mrs. Clinton’s grace and even humor under pressure were fully apparent during last year’s Benghazi hearing. Mr. Trump’s whiny braggadocio has been obvious every time he opens his mouth without reading from a teleprompter.

So how could someone like Mr. Trump have been in striking position for the White House? (He may still be there, since we have yet to see what effect the debate had on the polls.)

Part of the answer is that a lot more Americans than we’d like to imagine are white nationalists at heart. Indeed, implicit appeals to racial hostility have long been at the core of Republican strategy; Mr. Trump became the G.O.P. nominee by saying outright what his opponents tried to convey with dog whistles.