A new poll of Iowa Democrats shows three 2020 candidates inching ahead of former vice president Joe Biden, signaling a tightening contest for the Democratic nomination.

A Quinnipiac University poll released Wednesday showed 20 percent of voters support Sen. Elizabeth Warren, D-Mass., followed by 19 percent supporting South Bend, Ind., Mayor Pete Buttigieg, 17 percent for Sen. Bernie Sanders, I-Vt. and 15 percent for Biden. The poll has a 4.5 percentage point margin of error.

Just three months shy of the Iowa caucus, Quinnipiac University Polling Analyst Mary Snow described the heated competition as "a close race with a crowded field of frontrunners."

"While Senator Elizabeth Warren, former Vice President Joe Biden, and Senator Bernie Sanders have held top tier status in national polls for months, South Bend Mayor Pete Buttigieg joins their ranks in the 2020 Iowa caucus. And it's a race that is up for grabs. Half of likely Democratic caucus-goers who support a candidate say they may change their minds before Feb. 3," Snow added.

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Although Biden remains in the clear top tier of candidates, his slump in the polls both in Iowa and nationally has paved the way for other frontrunners to inch closer to him, and provide tight competition in the earliest primary states.

Fifty-two percent of likely Democratic voters surveyed said they could still change their minds before the first nominating contest in the 2020 Democratic presidential primaries, while 46 percent of voters said their mind is already made up.

The poll also showed that Sanders supporters were most committed to their chosen candidate, with 61 percent saying their minds were made up, compared to 48 percent of Biden voters, 44 percent of Warren voters and 40 percent of Buttigieg supporters.

Of the Democratic caucus-goers polled, 33 percent said they are looking for "someone who can win the general election."

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"Likely caucus-goers who selected electability as most important are divided in their top choice in the caucus, with Biden, Buttigieg, and Warren each getting 21 percent," Snow said. "There is no one candidate with a clear edge."

Democrats who described themselves as “very liberal” were divided between Sanders and Warren as their top picks, as they garnered 32 and 30 percent of the votes respectively. Those who describe themselves as “somewhat liberal” are divided between Warren at 29 percent and Buttigieg at 24 percent.

Buttigieg and Biden won favor with “moderate and conservative” Democrats, polling at 19 and 18 percent respectively.

Voters identified health care is the top issue in 2020, with 26 percent saying Sanders is strongest on the issue and 22 percent saying Warren.

Both progressive candidates have largely agreed on the idea of a single-payer "Medicare-for-all" health care system, but have recently butted heads on how to fund such a plan.

Thirty-five percent of Sanders supporters from the 2016 caucus say they still support him while 23 percent of his former voters say they now support Warren and 19 percent said they support Buttigieg. Biden had the backing of 27 percent of Hillary Clinton's 2016 caucus supporters, while 21 percent said they now support Buttigieg and 18 percent say they would vote for Warren.

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The survey polled 698 likely Iowa Democratic caucusgoers between Oct. 30 and Nov. 5.

Outside of the main contenders, Sen. Amy Klobuchar, D-Minn. got 5 percent of the vote, Sen. Kamala Harris D-Calif. got 4 percent, and businessman Tom Steyer, tech entrepreneur Andrew Yang and Rep. Tulsi Gabbard, D-Hawaii, each got 3 percent.