Equally important to the future of progressives in the Democratic Party is Mr. Sanders’s strength in the white working-class areas where Mr. Bradley, Mr. Obama, and both Mr. Brown and Mr. Tsongas faltered. It was Mr. Sanders’s strength among these voters that let him stay fairly competitive, even though he lost half of the traditional left-liberal coalition.

Mr. Sanders won white voters without a college degree by a double-digit margin in Connecticut, as he did in Maryland, Wisconsin, North Carolina, New Hampshire, Illinois, Oklahoma, Indiana, Vermont and Michigan. He probably did so in Rhode Island as well (no exit polls were conducted there).

Outside the South, Mrs. Clinton probably won white voters without a college degree only in Ohio (the exit polls there show she prevailed with that group by one point).

One possible explanation, again, is policy. Income inequality has become a vastly more important issue to Democrats since the Great Recession, and it’s reasonable to assume that white working-class Democrats might be especially drawn to the issue. This is the best case for the progressive left; it would mean that a future progressive populist could count on similar levels of support with a strong, class-oriented message.

The evidence for this view is somewhat mixed. According a compilation of exit polls, around 40 percent of white voters without a college degree wanted more liberal policies than those of Mr. Obama, and Mr. Sanders won these voters handily. The highest number was in Vermont, where 46 percent of white voters without a degree wanted more liberal policies than Mr. Obama’s.

That’s a big bloc that progressives can count on in the future, but it’s not a majority and it’s less than Mr. Sanders’s share of white voters without a degree. That’s in part because Mr. Sanders also won among those white working-class voters who wanted less liberal policies than those of Mr. Obama, a fact that makes Mr. Sanders look as much like a protest vote against Mrs. Clinton as the harbinger of a new Democratic socialism.

But it is nonetheless striking that so many white Democrats without a degree wanted more liberal policies than Mr. Obama’s. In fact, white voters without a college degree were often more likely than either college-educated white voters or minorities to support more liberal policies.