“There’s two ways people get inspired, in my experience,” Mr. Biden told voters in Newton during a wandering answer about climate change. “One, by really inspirational people like the John Kennedys of the world or the Abraham Lincolns of the world. And others by really lousy leaders.”

The lousy leader in this formulation seemed intuitive enough. Less clear was whether Mr. Biden had just conceded that the Democrats on offer were no Kennedys or Lincolns.

Seated in back, David Moseley, 72, said he had traveled to Iowa from Seattle to assess his options in person. He took his place in a gathering heavy on gentle applause and precarious digression as Mr. Biden moved through his remarks with a signature medley of “not a joke” interjection and “Barack and I” reminiscences.

“We don’t have a candidate that fits the entire coalition that we need,” Mr. Moseley ruled.

Other than that, Mrs. Lincoln, the play is going great.

Of course, much of the campaign strain has been born of healthy political combat, an ongoing debate over the party’s direction and purpose. Mr. Biden and Mr. Buttigieg have argued that a big-tent enterprise requires consensus and restraint. “Let’s not choose between boldness and unity,” Mr. Buttigieg told voters in Ottumwa on Tuesday, suggesting that he was offering both. “Let’s not choose between the right way to govern and the best way to win.”

Mr. Sanders, the leader in recent Iowa polls, and Senator Elizabeth Warren have spoken with more urgency, insisting that the scale of the country’s ills demands significant intervention, a zeal that has informed the electricity of their rallies.

“Kickass women win,” Ms. Warren said to cheers late Friday evening, thanking her female surrogates after arriving in Des Moines during a break from impeachment duties.