The sudden flurry of attention represents a change from earlier this year, when White House hopefuls appeared to be embracing a more national campaign strategy that took them beyond Iowa and other early nominating states. The growth of social media, the changes to the electoral calendar and the increasing diversity of the Democratic electorate combined to make it seem like Iowa, a predominantly white state, might lose its sway in the primary process.

“Usually there’s a ramp-up period in past presidential cycles,” said Sean Bagniewski, the Democratic chair in Polk County, about the focus on Iowa. “This year, it was like a thunderclap.”

The stakes in Iowa for Mr. Sanders have personal and psychological dimensions. After his surprising performance in the caucuses in 2016, some aides said the state has taken on an almost mystical quality. A weak result could also indicate that he does not have the same influence with progressive voters as he used to.

Despite a series of disappointing polls in the state — a Suffolk University/USA Today poll released this week showed him in fourth place — allies are confident he is still in a position to do well.

Like it did in 2016, the Sanders campaign is betting on an extensive ground game that relies heavily on a vast network of volunteers that they hope will motivate other supporters to turn out on caucus night — including people who do not typically participate in the political process.

His aides believe that the technique, known as distributed organizing, will help motivate unlikely or first-time caucusgoers, especially those who were too young to participate in the last presidential caucuses; Latino voters, whose turnout rate in the caucuses is typically low; and working-class voters. The campaign sees high potential for victory particularly in and around college towns, like Ames and Iowa City, and in rural areas.

To solidify his base of support, he has spent hours rallying on college campuses in an effort to capture the support of young voters. He has courted labor support particularly in counties along the Mississippi River. And he has held multiple “Unidos con Bernie” events with Latino voters.