Senator Bernie Sanders has opened up a lead in Iowa just over a week before the Democratic caucuses, consolidating support from liberals and benefiting from divisions among more moderate presidential candidates who are clustered behind him, according to a New York Times/Siena College poll of likely caucusgoers.

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.

The changing fortunes of the leading candidates underscores the volatile nature of the primary after more than a year of campaigning, as voters wrestle with which candidate can defeat President Trump. Despite the ascent of Mr. Sanders, a self-described democratic socialist, the poll also highlighted the combined appeal of the centrists: 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.”

As the strength of the other leading candidates has ebbed and flowed, Mr. Sanders, making his second run for the White House, appears to be peaking at the right time. This month was the first time he has finished atop a poll in Iowa, after also leading a Des Moines Register-CNN survey two weeks ago. The Times-Siena poll’s margin of error was plus or minus 4.8 percentage points.