One week before the opening Iowa caucuses, former Vice President Joe Biden leads a fluid Democratic field, a new Suffolk University/USA TODAY Network Poll of the state finds.

In the survey taken Thursday through Sunday, Biden was backed by 25% of likely Democratic caucusgoers, ahead of Vermont Sen. Bernie Sanders at 19%; former South Bend, Indiana, Mayor Pete Buttigieg at 18%; Massachusetts Sen. Elizabeth Warren at 13%; and Minnesota Sen. Amy Klobuchar at 6%.

But an additional 13% were undecided, and 45% of those with a preference said it was still possible they might change their minds. That underscores the possibility of more shifts in a state where Biden, Sanders, Warren and Buttigieg have each been ahead in one poll or another in recent months.

The Iowa outcome typically narrows the presidential field – costing also-runs the funds and standing to continue their campaigns – while providing a burst of momentum for the winner heading into the New Hampshire primary and other contests that follow.

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"Joe Biden continues to lead because the number one issue of caucusgoers is to defeat Donald Trump," said David Paleologos, director of Suffolk's Political Research Center. The former vice president "positions himself as a safer choice while the other three contenders fight it out below him."

Trump's impeachment trial may have reinforced the focus of Democratic voters on electability.

Two-thirds of those polled said they have watched at least some of the trial that began last week. In the end, 77% predicted the Senate won't convict the president and remove him from office.

Even so, they didn't view the impeachment proceedings as a waste of time. Seventy-one percent said the trial was "important" even if Trump was acquitted. Nearly 1 in 4, 23%, said the impeachment proceedings had made them more likely to participate in the caucuses, a sign that it may be energizing Democratic-leaning voters.

By 57%-31%, those surveyed said that "in their gut" they thought Trump would be defeated for reelection.

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Asked which issue was most important to them, nearly four in 10 said "defeating Donald Trump," almost double the 21% who identified health care. Only climate change also was cited by double digits, at 12%.

Most Biden supporters, 51%, called defeating Trump their top priority, compared with 34% of Sanders supporters and 29% of Warren supporters. Sanders led among voters who cited health care as their top issue. Warren and Klobuchar led among those who named climate change.

The poll of 500 likely Democratic caucusgoers, taken by landline and cellphone, has a margin of error of plus or minus 4.4 percentage points.

The support for Warren and Sanders was the most solid; about 6 in 10 of their backers said their minds were firmly made up. In contrast, a majority of Buttigieg and Klobuchar supporters said it was possible they would change their minds. Biden supporters by 53%-43% said their minds were made up.

Nearly two-thirds of the fieldwork for the poll was completed before The Des Moines Register announced its endorsement of Warren Saturday evening, a factor that could boost her standing in the state.

The Suffolk/USA TODAY Poll, taken Jan. 23-26, is the most current in a string of recent Iowa polls. A New York Times/Siena poll taken Jan. 20-23 showed Sanders leading at 25%, Buttigieg at 18% and Biden at 17%. A CBS News/YouGov poll taken Jan. 16-23 put showed Sanders at 26%, Biden at 25%.

The differing results could reflect changes in public sentiment or simply the effects of the margins of errors in polls.

In the new poll, entrepreneur Andrew Yang was at 3%, environmental activist Tom Steyer at 2% and Hawaii Rep. Tulsi Gabbard at 1%.

Biden did best among the oldest voters; 41% of those 65 and older supported him; Warren was second with just 11%. Sanders did best among the youngest voters; 33% of those 35 and under backed the Vermont senator; Buttigieg was second at 18%. Biden led among voters who said they were moderates; Sanders led among those who identified themselves as liberal or very liberal.

Meanwhile, there was some reassuring news for Sanders, Warren, Klobuchar and Michael Bennet – candidates whose day jobs as U.S. senators have required them to spend the campaign's final push sitting in the Senate chamber for Trump's trial rather than campaigning in Iowa. Eighty-eight percent said the senators' absences wouldn't affect their vote. Still, 5% agreed with the statement, "I expect candidates to be in Iowa to earn my vote."

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