Former Vice President Joe BidenJoe BidenThe Hill's Morning Report - House prosecutes Trump as 'lawless,' 'corrupt' What to watch for on Day 3 of Senate impeachment trial Sanders wants one-on-one fight with Biden MORE has extended his lead over progressive rivals Sens. Elizabeth WarrenElizabeth Ann WarrenTensions between McConnell and Schumer run high as trial gains momentum Sanders wants one-on-one fight with Biden Biden, Sanders tax plans would raise less revenue than claimed: studies MORE (D-Mass.) and Bernie SandersBernie SandersThe Hill's Morning Report - House prosecutes Trump as 'lawless,' 'corrupt' Tensions between McConnell and Schumer run high as trial gains momentum Sanders wants one-on-one fight with Biden 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 ButtigiegPeter (Pete) Paul ButtigiegThe Hill's Morning Report - House prosecutes Trump as 'lawless,' 'corrupt' Sanders wants one-on-one fight with Biden Overnight Defense: Trump downplays troops' concussion injuries in Iran attack | Dems offer case against Trump on day two of trial | UN links Saudis to hack of Bezos' phone MORE tied Sen. Kamala HarrisKamala Devi HarrisCalifornia Democrat Christy Smith launches first TV ad in bid for Katie Hill's former House seat Steyer spokesperson: 'I don't think necessarily that Tom has bought anything' Biden wins endorsement of Sacramento mayor 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 YangAndrew YangBiden leads Sanders by 7 in new national poll Sanders joins Biden atop 2020 Democratic field: poll Sanders holds 4-point lead on Biden in new California poll 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 BookerCory Anthony BookerThe Hill's 12:30 Report: House managers to begin opening arguments on day two Patrick backs reparations in unveiling 'Equity Agenda for Black Americans' Booker ahead of Trump impeachment trial: 'History has its eyes on us' MORE (D-N.J.), Amy KlobucharAmy Jean KlobucharBiden leads Sanders by 7 in new national poll Sanders joins Biden atop 2020 Democratic field: poll The Hill's Morning Report - Trump trial begins with clashes, concessions 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