Besides having supporters who skew older, the Biden campaign has fewer activists like Mr. Corken, according to Iowa Democrats. His campaign also appears to have recruited fewer precinct captains — the team leaders in each of more than 1,600 caucus sites — who play a key role wooing undecided voters.

Ann Fields, the Democratic chairwoman of Marion County, said that the campaigns of Senator Elizabeth Warren and of Pete Buttigieg, the former mayor of South Bend, Ind., had recruited caucus captains for all 17 precincts in her county southeast of Des Moines. Not so the Biden team. “I would say if they’re half full they’re lucky,” she said last week.

In Des Moines County in eastern Iowa, Tom Courtney, the Democratic co-chairman, said last week he knew of “only a few” Biden precinct captains in his 16 precincts. Asked about the likelihood that a candidate’s supporters would materialize on their own without an organizing staff, Mr. Courtney said, “I’ve not seen it before.”

His wife, Nancy Courtney, who is a Biden activist, said there was a “slim chance” Mr. Biden wouldn’t reach viability in some caucuses, meaning his support would fall below a 15 percent threshold needed to earn delegates.

“A lot of the campaigns have really good staffers and we only have one staffer in Des Moines County,” she said. “That worries me.’’

Pete Kavanaugh, Mr. Biden’s deputy campaign manager, said the candidate had plenty of staff members, both paid and volunteer, to turn out supporters.

“From the moment Vice President Biden announced his campaign, organizers and volunteers have been engaging Iowans in every corner of the state — building a robust, grass-roots organization that will turn out our committed supporters on caucus night,” he said. “We will have an Iowan as a precinct leader representing the campaign and making the case for Joe Biden in all of Iowa’s 1,678 precincts.”