The race is mostly unchanged since Monmouth’s previous poll, conducted two weeks earlier. In that survey, Biden held a slightly larger lead over Sanders, 24 percent to 18 percent, with Buttigieg at 17 percent and Warren at 15 percent — though all changes are well within the poll’s margin of error.

Biden draws the greatest support from older and more moderate voters, the poll shows, while Sanders’ base is younger and more liberal. Sanders is the first choice of 39 percent of likely caucus-goers under the age of 50, with Biden in fourth place at 7 percent. But among voters 65 or older, Biden is at 37 percent, and Sanders is in fifth place at only 9 percent.

Among poll respondents who say they are “very liberal,” Sanders leads Warren, 41 percent to 23 percent. Biden, meanwhile, is the top pick of a third of respondents who say they are “moderate” or “conservative,” 33 percent, with Buttigieg (17 percent) and Klobuchar (13 percent) trailing behind.

The Monmouth University poll was conducted Jan. 23-27, surveying 544 likely Iowa Democratic caucus-goers. The margin of error is plus or minus 4.2 percentage points.

As in other polls, the Monmouth survey shows the leading candidates bunched close together, with Biden and Sanders slightly ahead of the rest of the field. The race is still changeable, however: 45 percent of likely caucus-goers say they are open to supporting a different candidate next Monday, roughly the same share as the 47 percent who say they are firmly decided on a candidate.

Sen. Bernie Sanders arrives at a campaign stop in Des Moines on Jan. 20. | Andrew Harnik/AP Photo

But the topline result only measures voters’ initial preferences. The caucus process — which involves expressing public support for candidates in two rounds of voting — can change the result, eliminating the lower-polling candidates and giving their supporters a chance to pick one of the viable remaining hopefuls.

The Monmouth poll suggests six candidates are in danger of falling short of the viability threshold in many precincts: businessman Tom Steyer (4 percent), entrepreneur Andrew Yang (3 percent), Sen. Michael Bennet (1 percent), Rep. Tulsi Gabbard (1 percent), former Massachusetts Gov. Deval Patrick (0 percent) and former Rep. John Delaney (0 percent). Another 5 percent of likely caucus-goers say they are undecided.

The survey attempts to mimic the caucus process, first by asking respondents to pick their second choice. On that metric, Warren, the Massachusetts senator who’s slid to fourth in most Iowa polls, leads with 19 percent, followed by Biden (16 percent), Buttigieg (13 percent), Klobuchar (12 percent) and Sanders (11 percent).

Then, interviewers asked respondents to pick among only six candidates: the top five, plus Yang. On that ballot, Biden and Sanders are deadlocked at 22 percent, with Buttigieg (17 percent), Warren (16 percent), Klobuchar (12 percent) and Yang (5 percent) trailing behind.

Biden has a slight advantage when the race is boiled down to the top four candidates: The former vice president is at 29 percent, with Sanders at 25 percent, Buttigieg at 20 percent and Warren at 19 percent.

While Biden has a slim advantage in this poll, other recent surveys show Sanders ahead, including a New York Times Upshot/Siena College poll released last weekend. An analysis published Sunday by the Times found that Sanders generally leads in polls of all voters who say they are likely to caucus next week, while Biden runs better in surveys that are restricted to voters who have participated in recent elections.

The Monmouth poll is in the latter category: In order to be included in the sample, voters must be registered Democrats or independents who have voted in one of the two most recent primaries (2016 or 2018), the 2018 midterm election — or be newly registered since the 2018 election.

In a press release, Monmouth said it “examined what the race could potentially look like under different turnout scenarios” and found a similar phenomenon to the Times analysis. Biden runs better among voters who participate in primaries.

“An electorate that includes higher numbers of traditional Democratic primary voters could increase Biden’s lead to as much as 6 points,” the Monmouth release says. “On the other hand, a caucus electorate that is more heavily populated by voters who do not participate in primary elections could result in a Sanders lead of about 4 points.”