GARNER, Iowa — Dawn Smallfoot put up a Bernie Sanders sign in her yard after hearing him speak in spring 2015. It’s been there ever since.

“Why take it down?” she said on a recent Monday evening, during a break from making calls to potential Sanders supporters. “I was waiting for his return.”

His campaign is counting on that kind of devotion.

With less than six weeks until voting begins, the loyalty Mr. Sanders commands has turned him into a formidable contender in the 2020 race. Despite having a heart attack in October that threatened to derail his second quest for the Democratic nomination, he remains at or near the top of polls in Iowa and other early states, lifted by his near ubiquitous name recognition and an enviable bank account.

His anti-establishment message hasn’t changed for 50 years, and it resonates with working-class voters and young people who agree the system is corrupt and it will take a revolution to fix it.

The scenario seemed unlikely just months earlier. As Mr. Sanders, 78, lay recovering in a hospital in Las Vegas, two new stents in one of his arteries, some of his staff members were unsure if he would continue his campaign. With Mr. Sanders, Vermont’s junior senator, already slumping in the polls, even some allies thought he should drop out and throw his support behind Senator Elizabeth Warren of Massachusetts, a fellow progressive who was surging.

But then he secured the coveted endorsement of Representative Alexandria Ocasio-Cortez, Democrat of New York, giving his campaign a much-needed shot of energy. In the debates, he was steady, loose and largely unscathed. On the trail, he began to display a newfound joy and humor. And Ms. Warren slipped from the top of the field, reopening the progressive lane for him.

Mr. Sanders’s revival has reshuffled the Democratic primary race, providing a counterweight to the shift toward centrism in recent months that has elevated Mayor Pete Buttigieg of South Bend, Ind., and kept former Vice President Joseph R. Biden Jr. atop the national polls. And if it lasts, it would add to the likelihood of an extended primary battle, with Mr. Sanders splitting delegates in the early states with several other candidates.

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.

“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.”

Mr. Sanders has also mostly escaped aggressive attacks from his rivals. Other candidates have focused more on trying to stop Ms. Warren, whom they viewed as a bigger threat. On the airwaves, Mr. Buttigieg, another front-runner, has run television ads that attack Mr. Sanders’s proposals like “Medicare for all” and tuition-free public college but do not name him directly.

Ms. Warren herself has rarely criticized Mr. Sanders. Asked at a recent stop in the blue-collar town of Ottumwa what made her a “better candidate” than Mr. Sanders, she responded tepidly that they had been “friends for a long, long time.”

And though Mr. Sanders’s detractors see a numbing repetition in his message, his supporters see his constancy as one of his biggest assets: Mr. Sanders, for instance, has absorbed much less criticism on Medicare for all because he has championed it for decades. Ms. Warren’s evolving position on how to pay for it has hurt her with some voters.

During a recent rally in Burlington, a town along the Mississippi River in southeastern Iowa, Mr. Sanders played his greatest hits. Standing behind a podium, he railed against income inequality. He trumpeted health care as a human right.

“What this campaign is about is trying to talk about issues, and bring about ideas that address the pain of working families in this country,” he said. The audience nodded along. Many had heard it before. Many had come to hear it again.

“I do like that he has fought for people the same his entire Senate career and even before,” said Angel Edwards, 41, of Burlington. “It makes you hopeful that he won’t flip-flop while in office.”

Since his heart attack, Mr. Sanders has often seemed lighter and more relaxed, a change from the gruff intensity that for years marked his public appearances. His campaign frequently posts videos of him shooting hoops, and he recently took a few pitches of batting practice at an event at an indoor sports facility in Burlington.

At the same time, there is little indication, in Iowa and elsewhere, that Mr. Sanders is attracting more supporters beyond those who backed him in 2016 and young people who were not old enough to vote then. In interviews with dozens of people at his campaign events in recent months, nearly all said their support dated to his first presidential run, or earlier; at events for other candidates, hardly anyone mentions Mr. Sanders as a top choice.

“From my conversations, it appears that people are not ambivalent about Sanders,” said Jeff Fager, the Democratic chairman in Henry County, where Mr. Sanders battled Hillary Clinton to a tie in 2016. “They are either behind him, or he is not on their list of potential candidates.”

That steady support could be enough in Iowa, whose complex caucus system favors on-the-ground enthusiasm, especially if excitement for other candidates wavers. The challenge, however, is that Mr. Sanders is effectively gambling that those who do not usually vote will now show up on a cold Monday night in February to participate in what could be an hourslong, sometimes disorganized process.

Kurt Meyer, the Democratic chairman in Mitchell County, in northern Iowa, said he saw signs that Mr. Sanders’s organization might have trouble turning out potential caucus attendees in his rural region.

“The Sanders organization in the predominantly rural counties I am most familiar with is not particularly strong,” he said. While he suggested that it might be easy to underestimate the totality of Mr. Sanders’s support, he also said it was far from clear that the voters Mr. Sanders was counting on would show up on caucus day.

Aides to Mr. Sanders provide few details on how they are wooing supporters, but they express confidence that their strategy is working. The campaign said in late October that it already had more commit-to-caucus cards, a loose measure of support, than it did in January 2016.

Just as it did that year, Mr. Sanders’s team is trying to connect with people in new ways. His campaign canvasses at farmers’ markets and outside drugstores. One field organizer, Conrad Bascom, has started holding phone banks at a Casey’s General Store in rural Garner largely because the location was a convenient meeting place and the Wi-Fi was reliable.

“It happened pretty organically,” Mr. Bascom said, during one such phone-bank event in early December. “It very quickly became a habit.”