But first is Iowa, where the race remains up for grabs to an unusual degree so late in the race: In the Times poll, nearly 40 percent of voters said they could still be persuaded to caucus for a different candidate.
Mr. Sanders, however, has some of the most committed supporters in the race and, significantly, his advantage with Iowa Democrats does not ebb when the field is narrowed to only the top four candidates. The Vermont senator still leads by seven points in that scenario, according to the survey, capturing 30 percent of the vote while Mr. Biden and Mr. Buttigieg both win 23 percent and Ms. Warren garners 19 percent.
These results are important because of how the caucuses work in Iowa. Candidates must receive 15 percent support in a precinct to reach what is called viability, meaning they are eligible to win delegates. If a candidate falls short of that number, his or her supporters can either leave the caucus or support better-performing contenders for the second and final vote of the evening. So voters’ second choices can prove crucial.
Ms. Warren, according to the poll, is the top second-choice preference of caucusgoers, which could lift her candidacy after the initial vote. But that is in part because she is the preferred alternative for those who support Mr. Sanders, who will most likely meet the viability threshold in nearly all of the caucuses.
Ms. Klobuchar’s supporters could play the most crucial role, though, should she fail to achieve 15 percent in most precincts. When voters were asked whom they would support if they were left with only four choices — Mr. Sanders, Mr. Buttigieg, Mr. Biden and Ms. Warren — Mr. Biden was the overwhelming favorite of Ms. Klobuchar’s backers. He received 55 percent of them in this scenario while Ms. Warren received 18 percent.
One troubling sign for Mr. Sanders came in an ideological matchup with Mr. Trump. In The Times/Siena poll, 56 percent of caucusgoers said they thought a democratic socialist would have a harder rather than an easier time defeating the president — a higher number than those who said the same about a woman, a gay candidate or one over 75.
In fact, among general election voters in Iowa, a state he carried by nearly 10 points four years ago, Mr. Trump defeated all of the top five candidates as well as Mr. Bloomberg in head-to-head matchups. He bested Mr. Sanders, 48 percent to 42 percent.