He still faces a difficult path to the nomination. Ms. Warren has siphoned off some of his support, and his entire campaign rests on the conviction that he can pull in voters who might otherwise not show up at the ballot box.

In addition, he has a strained relationship with the Democratic establishment, which remains bitter over the division he and his supporters sowed after the 2016 primaries, and chafes at his refusal to engage with the traditional party apparatus.

Yet in Iowa, and elsewhere, the tension with the party has served only to re-energize Mr. Sanders and his loyalists, who are faithful to him in a way that no other candidate’s supporters are: While backers of other Democrats often list three or four contenders when asked to name their top choice, Mr. Sanders’s fans are unwavering.

A recent poll from The Des Moines Register showed that, among likely Democratic caucusgoers who said Mr. Sanders was their top choice, 57 percent said their minds were made up; according to The Register, no other candidate registered above 30 percent.

Those figures alone could portend a strong showing for Mr. Sanders at the caucuses, where candidates must receive at least 15 percent support at a caucus site to collect that site’s share of state delegates.

“Bernie Sanders is definitely being underestimated in Iowa,” said John Grennan, the Democratic chairman in Poweshiek County, Iowa.

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“Part of his durability is that he has 15 to 20 percent of the caucus who are absolutely committed to voting for him no matter what,” he said. “In a field that’s split between at least 10 major candidates, that 15 to 20 percent counts for a whole heck of a lot.”