From the moment Donald Trump announced his candidacy for the White House until very late on Election Night 2016, media and pollsters kept insisting Trump wouldn't--couldn't--be president.

But for Larry Schweikart-one of a ragtag group of amateur politicos called "the Deplorables" who had been publishing shockingly accurate polls and predictions-and Joel Pollak-a Breitbart News senior editor following Trump on the campaign trail-Trump's win was a near certainty. Schweikart and Pollak watched the Trump campaign build a powerful coalition between working Americans from both parties;...