As the clear front-runner, Trump is drawing support from across the GOP as the critical South Carolina primary approaches on Saturday. But, from the start, his strength has been rooted in his extraordinary hold on Republicans without a college education. In the crowded New Hampshire primary, he won fully 42 percent of them, more than his next three closest competitors combined, according to exit polls.

Whites without a college degree, who began realigning away from the Democratic Party in the 1960s, are arguably now the GOP’s most important constituency. In every presidential election since 1996, they have provided 49 to 55 percent of the votes won by the GOP presidential nominee—even though non-college whites over that period have shrunk from about half of all voters to only about one-third. They played equally crucial roles in the 2008 and 2012 Republican presidential primaries, casting about half the votes.

Yet no GOP presidential contender more formidable than the conservative commentator Patrick J. Buchanan in 1996 or former Senator Rick Santorum in 2012, neither of whom came close to winning, has aimed his agenda squarely at those voters. In style and substance, John McCain and Mitt Romney, the 2008 and 2012 nominees, more represented the party’s managerial white-collar wing. Trump has decisively broken that pattern with a bristling, insular message that attacks both domestic elites and foreign influences, from Mexican immigrants to Chinese manufacturers.