Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.) led former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE in Iowa for the first time in a presidential primary poll released Saturday.

The Des Moines Register/CNN/Mediacom poll of likely Democratic caucusgoers in Iowa found that 22 percent support Warren for president, while 20 percent said Biden was their top choice.

The two top candidates were within the poll's margin of error, but it represents the first time Biden hasn't been at the top of the field in the crucial first-in-the-nation caucus state.

When considering second choices, Warren's lead over Biden expands, with a total of 42 percent of likely Democratic voters calling her their first or second choice. Biden was one of the top two choices for 30 percent of likely Democratic voters.

In the Iowa caucus, a voter’s first choice is viable only if they have at least 15 percent of the vote. If they don't, the voter can switch to their second choice.

The top two candidates were trailed by Sen. Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE (I-Vt.) with 11 percent, South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE with 9 percent and Sen. Kamala Harris Kamala HarrisTexas Democratic official urges Biden to visit state: 'I thought he had his own plane' The Hill's Campaign Report: Biden on Trump: 'He'll leave' l GOP laywers brush off Trump's election remarks l Obama's endorsements A game theorist's advice to President Trump on filling the Supreme Court seat MORE (D-Calif.) with 6 percent.

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Biden was ahead of Warren in the Des Moines Register's three past polls of the 2020 cycle, but Warren has seen a steady increase.

In December, she had the support of 8 percent of respondents, while she had the support of 9 percent in March. In June, 15 percent of respondents supported her, although the paper noted that the June poll had a slightly different methodology than the others. The Massachusetts senator has also seen increased momentum in recent months on a national level.

J. Ann Selzer, president of Selzer & Co., which conducted the poll, told the Des Moines Register that this was the race's “first major shakeup.”

“It’s the first time we’ve had someone other than Joe Biden at the top of the leader board,” Selzer said.

Researchers surveyed 602 likely Democratic caucusgoers between Sept. 14 and Sept. 18. The poll's margin of error is 4 percentage points.

Biden has been the front-runner of the 2020 Democratic presidential race in most major polls, although Warren's campaign has seen momentum in recent months.

They are among more than a dozen people running for the party's nomination.