Bernie Sanders’s coalition once looked very familiar: He had strong support among well-educated and affluent liberal white voters of the sort who backed Barack Obama, Bill Bradley and Jerry Brown. He struggled among less affluent voters.

But his coalition has evolved over the last few months. He now fares much better among less affluent whites than Mr. Obama did eight years ago, suggesting he’s attracting a group that traditionally supports more moderate establishment candidates, someone like Hillary Clinton. If confirmed in the voting, it would vindicate his hope of building a progressive coalition based more on class than the coalitions put together by liberal predecessors.

But these gains have come at a cost. There are signs he has lost Mr. Obama’s edge among affluent white voters in Iowa and New Hampshire. Affluent whites are one of the highest turnout groups in the country and traditionally a key ally of reformist, outsider candidates. And his increase in strength among white working-class voters makes more puzzling the question of why he struggles so much among nonwhite voters, who continue to back Mrs. Clinton by wide margins.

The evidence for Mr. Sanders’s strength among less affluent white voters, at least compared with Mr. Obama, is persuasive in both state and national polls.