A new national poll of Democratic primary voters helps illustrate why Mr. Sanders is so eager to pick a fight with Mr. Biden: The former vice president has gained ground since entering the race and enjoys the support of 39 percent of his party’s voters while Mr. Sanders is in second place with 15 percent.

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There are other reasons that Mr. Sanders is trying to frame the race in one-on-one terms. The same poll showed that Mr. Biden, a mainstream progressive, also enjoys a wide lead right now with liberals, with 32 percent compared with Mr. Sanders’s 19 percent. While the two men are seen by political insiders as hailing from starkly different wings of the party, here in Iowa, there is considerable overlap in their early bases of support.

A Des Moines Register-CNN survey last month of likely Iowa Democratic caucusgoers indicated that Mr. Biden had an overall advantage over Mr. Sanders within the margin of error, but more revealing is that their supporters named the other man as their preferred second choice.

That’s partly the result of their being the two best-known candidates at this early stage of the race. But the 2016 presidential primary, in both parties, demonstrated that assumptions about voters falling along expected ideological lines can be mistaken.

Longtime Iowa Democrats, however, note that there is another precedent that Mr. Sanders must be mindful of: The candidate who goes on the attack in the state is often not the one who reaps the benefit when the candidate being attacked drops in the polls.

“You’ve got to be careful because we don’t like that,” said Dave Nagle, a former Iowa Democratic chairman. He recalled how Richard A. Gephardt’s attacks on Howard Dean, the front-runner in the 2004 race, only helped John Kerry complete his stunning comeback. “I don’t think it’s helpful for the senator from Vermont to go after a combatant from the same army at this point,” Mr. Nagle said, referring to Mr. Sanders.

As for Mr. Biden, the only Democrat he is eager to discuss at the moment is the one who made him vice president.