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 has extended his lead over progressive rivals 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.) in the latest Hill-HarrisX poll released on Monday.

The poll of likely Democratic and independent voters found that 31 percent support Biden for president, a 4-point increase from the last time the poll was conducted in late August.

Sanders came in second with 16 percent, while Warren followed in third place with 14 percent.

Though still within the margin of error, this uptick in support for Biden is a positive for a campaign that is trying to hold off Warren.

A Des Moines Register/CNN/Mediacon poll released Saturday showed Warren overtaking Biden for the first time in the caucus state. Warren won 22 percent support in the poll, compared to 20 percent for Biden.

In the Hill/HarrisX poll, 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 tied 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.) for fourth place, with each winning 5 percent.

Former Texas congressman Beto O’Rourke (D) followed with 4 percent.

Former 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 dropped back to 2 percent after previously climbing to 5 percent.

The only other candidates to poll at least 2 percent or higher were 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.), Amy Klobuchar Amy Klobuchar3 reasons why Biden is misreading the politics of court packing Social media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight MORE (D-Minn.), and former Housing and Urban Development Secretary Julián Castro.

HarrisX researchers surveyed 440 Democratic and independent voters between Sept. 20 and Sept. 21. The margin of error for this poll is plus or minus 4.7 percentage points.

The Democratic field narrowed last week after New York City Mayor Bill de Blasio dropped out of the 2020 presidential race averaging in several national polls at less than 1 percent.

De Blasio, who first launched his bid in May, announced the news on MSNBC’s “Morning Joe,” and vowed to continue his work as mayor of New York City.

“It’s clearly not my time, so I’m going to end my presidential campaign, continue my work as mayor of New York City, and I'm going to keep speaking up for working people," he said at the time.

—Tess Bonn