Mr. Sanders has gained six points since the last Times-Siena survey, in late October, and is now capturing 25 percent of the vote in Iowa. Pete Buttigieg, the former mayor of South Bend, Ind., and former Vice President Joseph R. Biden Jr. have remained stagnant since the fall, with Mr. Buttigieg capturing 18 percent and Mr. Biden 17 percent.

The rise of Mr. Sanders has come at the expense of his fellow progressive, Senator Elizabeth Warren: she dropped from 22 percent in the October poll, enough to lead the field, to 15 percent in this survey. Senator Amy Klobuchar, who is garnering 8 percent, is the only other candidate approaching double digits…

Despite Mr. Sanders’s ascent, the combined strength of the moderate candidates is unmistakable. The poll showed that 55 percent of those surveyed said they preferred a standard-bearer who is “more moderate than most Democrats.” Just 38 percent said they wanted one who is “more liberal than most Democrats.”