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 holds a double-digit lead against his closest rivals for the Democratic presidential nomination.

But there are signs Biden's support may be slipping, according to a new Harvard CAPS/Harris Poll survey released exclusively to The Hill.

The poll showed Biden with 28 percent support among registered Democratic voters, a 4-point drop since last month when a similar survey showed him at 32 percent. He’s trailed by Sens. 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.) and 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.), who registered 17 percent and 16 percent, respectively.

No other candidate registered double digits. 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.), the fourth-place finisher in the poll, notched 6 percent, while three other candidates, 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, former Rep. Beto O’Rourke (D-Texas) and entrepreneur Andrew Yang Andrew YangBiden's latest small business outreach is just ... awful Doctor who allegedly assaulted Evelyn Yang arrested on federal charges The Hill's Campaign Report: Biden weighs in on police shootings | Who's moderating the debates | Trump trails in post-convention polls MORE, tied for fifth place with 3 percent each.

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Only three others, former Housing and Urban Development Secretary Julián Castro, Sen. Cory Booker Cory Anthony BookerThe movement to reform animal agriculture has reached a tipping point Watchdog confirms State Dept. canceled award for journalist who criticized Trump 3 reasons why Biden is misreading the politics of court packing MORE (D-N.J.) and billionaire philanthropist Tom Steyer Tom SteyerTV ads favored Biden 2-1 in past month Inslee calls Biden climate plan 'perfect for the moment' OVERNIGHT ENERGY: Trump administration finalizes plan to open up Alaska wildlife refuge to drilling | California finalizes fuel efficiency deal with five automakers, undercutting Trump | Democrats use vulnerable GOP senators to get rare win on environment MORE, registered above 1 percent in the poll, taking 2 percent support each.

The Harvard CAPS/Harris Poll surveyed 693 registered Democratic voters from Sept. 22-24, and does not report a margin of error.

While Biden still leads in the poll, there were positive signs for Warren, who gained 4 points since a Harvard CAPS/Harris Poll survey conducted last month. The Massachusetts senator saw her support among black voters, a key Democratic voting bloc, more than double, ticking up from 6 percent in August to 13 percent at the end of September.

She also saw a 6-point gain among white voters, jumping from 15 percent in August to 21 percent in September, the poll shows. Biden, meanwhile, lost 10 points among white voters, falling from 35 percent last month to 25 percent now.

“We confirm that Biden is holding on, but Warren is surging as she develops a national following within the primary electorate,” Mark Penn Mark PennThe Supreme Court vacancy — yet another congressional food fight Trump, Biden battle over rush for COVID-19 vaccine The 7 keys to victory in the presidential race MORE, the co-director of the Harvard CAPS/Harris Poll, said. “The Sanders campaign could be in real trouble if they do not stem the tide soon.”

The poll follows on the heels of a slew of other surveys showing Warren taking the lead from Biden. A Des Moines Register/CNN/Mediacom poll released last weekend showed her at 22 percent support, edging out Biden at 20 percent. And a Monmouth University poll found her leading Biden in New Hampshire, 27 percent to 25 percent.

Likewise, a Quinnipiac University national poll released on Wednesday showed Warren leading the pack with 27 percent support. Biden came in second place in that survey, with 25 percent support.

Still, the Harvard CAPS/Harris Poll survey released on Thursday showed that a plurality of voters — 34 percent — perceive Biden as the candidate with the best chance of defeating President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE in 2020. Sanders and Warren are statistically tied for second place on that front, scoring 12 percent and 11 percent, respectively.

The Harvard CAPS/Harris Poll surveyed 693 registered Democratic voters from Sept. 22-24. The poll is a collaboration of the Center for American Political Studies at Harvard University and The Harris Poll. The Hill will be working with Harvard CAPS/Harris Poll throughout 2019.

Full poll results will be posted online later this week. The Harvard CAPS/Harris Poll survey is an online sample drawn from the Harris Panel and weighted to reflect known demographics. As a representative online sample, it does not report a probability confidence interval.