Joseph R. Biden Jr. has regained some of his strength in Iowa, but his three closest rivals for the Democratic nomination remain clustered at his heels, according to a Monmouth University poll released on Monday.
The poll, published three weeks before Iowa’s first-in-the-nation nominating contest, found the former vice president with support from 25 percent of likely Democratic caucusgoers, a bump of five percentage points since Monmouth’s most recent Iowa poll, in November.
Mr. Biden’s three closest competitors are clumped together in a statistical tie, with Senator Bernie Sanders of Vermont earning 18 percent support, former Mayor Pete Buttigieg of South Bend, Ind., 17 percent, and Senator Elizabeth Warren of Massachusetts earning 15 percent. The poll had a five-point margin of error.
Mr. Sanders has leapt five points since the last Monmouth poll of the state, Mr. Buttigieg experienced a five-point drop, and Ms. Warren experienced a three-point drop.
The Monmouth poll’s results differ somewhat from those of a CNN/Des Moines Register poll released late last week that showed Mr. Sanders atop the Democratic field, with 20 percent support. That poll, which was taken in the days just before the Monmouth poll was conducted, found Mr. Biden at just 15 percent.
The Monmouth poll was taken from Jan. 9-12, of 405 likely caucusgoers. It found that a slim majority of Democratic voters still have not firmly decided on a candidate, meaning that there is still a great deal of fluidity in the state.
It was conducted before the departure of Senator Cory Booker of New Jersey, who dropped out of the race on Monday and who pulled 4 percent support in the poll.
Under the rules of the Iowa caucuses, many voters who support less popular candidates will have to back someone other than their top choice in the final balloting. Ms. Warren has strength in this regard: She is by far the most popular second-choice pick among voters, with 23 percent naming her as their personal runner-up, according to the poll.