Through the end of June, he had an estimated 7,000 individual donors in Iowa, according to an analysis of campaign fund-raising records by The New York Times, by far the most of any candidate. And he maintains an army of die-hard liberal foot soldiers who are more than eager to propel him to the convention. On Friday, before the state’s annual Wing Ding dinner in Clear Lake when supporters for the various candidates typically gather and chant outside the event as a show of force, Mr. Sanders’s team boasted that their volunteers had instead knocked on every Democratic door in the town. He plans to return to Iowa next week for what will be his eighth trip to the state since announcing his candidacy in February.

But the landscape for Mr. Sanders is vastly different than it was four years ago. Nearly two dozen candidates are now vying for the nomination. And unlike in 2016, when he had the liberal populist message to himself, there are now many other progressives who have adopted a similar agenda. There is also a surging energy among young activists for diversity, female candidates and generational change.

At the same time, several Iowa Democratic officials said they were miffed by Mr. Sanders’s campaign, which they see as operating as something of a lone wolf.

Jeannine Grady, Democratic chairwoman in Marshall County, where Mr. Sanders defeated Mrs. Clinton in the caucuses in 2016, said Mr. Sanders’s campaign is not following the traditional campaign playbook of staying in close contact with county chairs.