(Bloomberg) -- Joe Biden is on top in Iowa, as more voters say they have "firmly decided" on their choice, according to a Monmouth University poll released Monday.
Biden is ahead with 24% support, up 5 points from the same poll in November. Bernie Sanders is in second place with 18%, virtually tied with Pete Buttigieg at 17%, and with Elizabeth Warren at 15%.
Biden has been at at the top of most Iowa polls, though Sanders had the lead in a Des Moines Register/CNN/Mediacom poll released Friday, with 20%. Biden was fourth in that poll, with 15% support, behind Warren and Buttigieg.
The Monmouth poll is notable because 43% of respondents said they are "firmly decided" on their top choice, up from 28% in November. Still, that left more than half saying that they were at least open to the possibility of choosing another candidate.
The poll of 405 likely caucus goers was conducted Jan. 9-12 and had a margin of error of 4.9 percentage points.
In recent weeks, Biden has sought to revive a flagging operation in Iowa, spending more time in the state and outlaying more on online and TV ads ahead of the Feb. 3 caucuses.
Biden's front-runner status in the poll could cause him to come under fire from his rivals on Tuesday, as six Democratic candidates meet in Des Moines for the final debate before the caucuses. Along with Warren, Sanders and Buttigieg, he'll also face Minnesota Senator Amy Klobuchar and billionaire activist Tom Steyer.
The poll also revealed more about who voters would support as a second choice.
Under Iowa caucus rules, supporters of candidates who don't receive at least 15% on the first round have to switch to a viable candidate, so the second picks of supporters could prove important.
Warren led for second choice, with 23% saying they would pick her if their first choice didn't make the cut, up from 17% in November. Buttigieg was virtually tied with Sanders at 15% and 14% respectively, while Biden was the second choice of only 10%.
But when the question was phrased a different way, Biden came out on top.
Pollsters asked likely caucus goers who they would pick if the only viable candidates in their caucus site were the top four. Biden was at 28%, Buttigieg at 25%, Sanders at 24% and Warren at 16%.
New Jersey Senator Cory Booker, who suspended his campaign Monday, had just 3% support in the Monmouth poll. His departure shrank the field of Democratic contenders to 12.
(Michael Bloomberg is also seeking the Democratic presidential nomination. He is the founder and majority owner of Bloomberg LP, the parent company of Bloomberg News.)
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