He is also on the rise among voters of color. He has courted Hispanic voters especially and is now the clear favorite among them — a fact that owes partly to the relative youth of the Latino electorate. He is also increasingly popular among African-Americans, many of whom are facing a hard decision about whom to support, now that Mr. Biden’s star is fading.

If Mr. Biden continues to sink, Mr. Sanders may have a relatively open lane with voters of color, since Mr. Buttigieg, Ms. Warren of Massachusetts and Senator Amy Klobuchar of Minnesota have all had difficulty gaining traction there.

But Michael R. Bloomberg, the former mayor of New York, has bought over $350 million in television ads since the fall and is aggressively seeking out black and Hispanic voters. Among black voters, he is now polling about evenly with Mr. Sanders, according to the latest Quinnipiac poll, which found that they were each supported by about one-fifth of black Democratic voters.

Much remains in flux, and the results of the coming contests — in Nevada, South Carolina and the 14 states that will vote on Super Tuesday — will have a big impact on the rest of the race. But there are already some hopeful signs for Mr. Sanders.

Monmouth’s most recent poll of Democratic voters found that he enjoys 30 percent support among respondents living in states that will vote after Super Tuesday, versus just 21 percent among earlier voters. If he continues to do well enough to maintain front-runner status through Super Tuesday, on March 3, he may have a slightly easier map for the rest of the primary season.

Sean McElwee, the co-founder of the progressive think tank Data for Progress, said that Mr. Sanders’s high favorability ratings put him in a stronger position to win over moderate Democrats than many pundits acknowledged. He said that while Mr. Sanders’s campaign had yet to aggressively target more moderate voters, that didn’t mean it could not be done.

“It’s a question of when the campaign decides to do it, but he is undeniably at this point the front-runner, and it makes a lot of sense right now for him to begin moving beyond his base and start to aim toward the center,” Mr. McElwee said in an interview.

“Bernie Sanders has incredibly high favorability numbers among Democrats; voters overwhelmingly view him positively,” Mr. McElwee added. “I think the idea that he was unelectable was always a myth.”

Michael Venutolo-Mantovani contributed reporting from Durham, N.C.