But for now, some of his rivals seem less interested in taking advantage of the opening in Iowa than in exploiting his record on the debate stage. Many candidates are making a concerted effort in Iowa, but not all of the major candidates have been especially focused on the state.

Image The Iowa State Fair Butter Cow, 2018 version. Credit... Sam Hodgson for The New York Times

The cause of Mr. Biden’s weakness in Iowa is fairly obvious: His national edge is mainly attributable to a wide advantage among black voters, and relatively speaking, there aren’t many black voters in Iowa. African-Americans represented a mere 4 percent of Hillary Clinton’s 2016 supporters in Iowa, according to Upshot estimates. (They represented 22 percent of her support nationwide.) The latest Quinnipiac poll gives Mr. Biden a mere three-point lead nationally among white voters, who could make up around 90 percent of the caucus vote in Iowa.

The state’s Democratic voters are also relatively young, and caucuses tend to favor ideologically consistent, progressive candidates with highly engaged grass-roots support.

Most of this is true of New Hampshire as well. But because Vermont and Massachusetts are next door, Mr. Biden would have a convenient excuse if he lost to either Bernie Sanders (a Vermont senator) or Elizabeth Warren (a Massachusetts senator).

That story could start to look very different if Mr. Biden lost Iowa and New Hampshire. Historically, candidates who fall short in both states lose about half of their support in an average of national polls conducted between Iowa and Super Tuesday, compared with national polls taken after December of the prior year. A strong second-place finish might be enough to prevent a candidate from losing much support, especially if it were a well-known and established candidate like Mr. Biden. But plainly it would pose a substantial risk.