Donald J. Trump won the presidency by riding an enormous wave of support among white working-class voters.

It was always a possibility, but it had always looked unlikely. Hillary Clinton led in nearly every national poll — and in other surveys in the states worth the requisite 270 electoral votes.

The traditional view of recent American elections gave even more reason to think Mrs. Clinton was safe. National exit polls suggested that President Obama won the 2012 presidential election despite faring worse among white voters than any Democrat since Walter Mondale. Those polls showed that white voters without a degree were now just one-third of the electorate. It was interpreted to mean that there was not much room for additional Democratic losses, especially once a white Democrat replaced Mr. Obama on the ballot.

The truth was that Democrats were far more dependent on white working-class voters than many believed.