Everything seemed to be clicking for Mr. Sanders before last week. He had finished at the top of the nominating contests in Iowa and New Hampshire, then dominated in Nevada. But on Super Tuesday, a surging Mr. Biden all but extinguished that momentum, winning 10 of 14 states with the support of many black working-class voters. Mr. Biden and Mr. Sanders are now heading into primaries in Michigan and other major Midwestern states that are favorable in many ways to the former vice president. But Mr. Sanders also enjoys plenty of support in these states, particularly from white working-class voters.

In both of his bids for the White House, Mr. Sanders has shown that his populist message resonates in some corners, even as it repels much of the Democratic establishment, which has steadily lined up behind Mr. Biden. Rallies for Mr. Sanders often resemble rock concerts, drawing tens of thousands of people who come decked out in campaign gear, with T-shirts that proclaim “Unidos con Bernie” and signs that say “Not me, Us.”

Polling throughout the campaign has shown Mr. Sanders drawing some of his strongest support from voters with household incomes under $50,000; his numbers taper off as incomes rise. A month ago, when he was leading in the polls, people with household incomes of $50,000 and under supported Mr. Sanders twice as much as any other candidate. At that time, he commanded the support of most Democratic voters making $100,000 and under.

Exit polls on Super Tuesday did not ask respondents directly about their income. But in the three states where he won and exit polls were conducted — Colorado, Vermont and California — Mr. Sanders performed five to eight percentage points better among those without a college degree than those with one. In Massachusetts and Minnesota, both states he had hoped to win but ended up losing decisively to Mr. Biden, Mr. Sanders’s numbers among college graduates lagged his showing among those without degrees by double digits.

“Bernie is the only candidate I’ve ever felt a connection to, in a sense that he genuinely cares about the working class in a way that no other candidate has ever shown support to us,” said Andrew Hilbert, 26, who came to see Mr. Sanders in Phoenix.

Mr. Sanders’s support this year has proved particularly enduring in the West, where many communities remain visibly scarred by the Great Recession. And his focus on the working class helps explain part of his appeal to Latino voters, who are disproportionately young and are more likely to come from a working-class background. Many such voters point to the illusion of an “up by your bootstraps” mentality and strongly believe that the only way to create a fair economy is to drastically change the way the current one works.