Former Vice President Joe BidenJoe BidenSupport for impeachment inches up in poll Overnight Defense: Trump's Syria envoy wasn't consulted on withdrawal | McConnell offers resolution urging Trump to rethink Syria | Diplomat says Ukraine aid was tied to political investigations Democrats say they have game changer on impeachment MORE has extended his lead over progressive rivals Sens. Elizabeth WarrenElizabeth Ann WarrenOvernight Health Care — Presented by Partnership for America's Health Care Future — ObamaCare premiums dropping for 2020 | Warren, Buttigieg shift stances on 'Medicare for All' | Drug companies spend big on lobbying Mellman: Trumping peace and prosperity On The Money: Waters clashes with Trump officials over 'disastrous' housing finance plan | Dems jump into Trump turf war over student loans | House passes bill targeting anonymous shell companies MORE (D-Mass.) and Bernie SandersBernie SandersOvernight Health Care — Presented by Partnership for America's Health Care Future — ObamaCare premiums dropping for 2020 | Warren, Buttigieg shift stances on 'Medicare for All' | Drug companies spend big on lobbying Mellman: Trumping peace and prosperity Tlaib to join Sanders at campaign rally in Detroit 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 ButtigiegOvernight Health Care — Presented by Partnership for America's Health Care Future — ObamaCare premiums dropping for 2020 | Warren, Buttigieg shift stances on 'Medicare for All' | Drug companies spend big on lobbying Poll: Biden holds 2 point lead over Sanders nationally Saagar Enjeti: Warren, Buttigieg don't stand a chance against Trump MORE tied Sen. Kamala HarrisKamala Devi HarrisSaagar Enjeti: Warren, Buttigieg don't stand a chance against Trump Warren overtakes Sanders in new poll The Hill's 12:30 Report: Trump faces backlash for comparing impeachment to 'lynching' 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 YangSaagar Enjeti: Warren, Buttigieg don't stand a chance against Trump Warren overtakes Sanders in new poll Poll: Biden leads Warren by 15 points in California 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 BookerProgressive commentator: Voters becoming weary of Warren policy proposals Saagar Enjeti: Warren, Buttigieg don't stand a chance against Trump The Hill's 12:30 Report: Trump faces backlash for comparing impeachment to 'lynching' MORE (D-N.J.), Amy KlobucharAmy Jean KlobucharHillicon Valley: Zuckerberg would support delaying Libra | More attorneys general join Facebook probe | Defense chief recuses from 'war cloud' contract | Senate GOP blocks two election security bills | FTC brings case against 'stalking' app developer Senate Republicans block two election security bills Warren overtakes Sanders in new poll 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