A new poll shows that 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’s supporters are starting to warm up to 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.) as a viable alternative, though 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.) is still the preferred second choice.

The Hill-HarrisX poll released on Monday found that 20 percent of likely Democratic voters who support Biden would support Warren as their second choice, which marks a 12-point increase from when an identical poll was conducted in May.

However, Sanders still remains the top second-choice candidate among Biden supporters at 35 percent, an 8-point increase from the May 3-4 poll.

Another 10 percent of Biden supporters said they would back 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.), a 5-point dip from the last poll. Former Rep. Beto O’Rourke (D-Texas) garnered 5 percent support, a 6 point fall. 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 (D) followed, dipping to 7 percent from 10 percent.

Other polls have shown similar results.

According to a weekly Morning Consult data tracker, 25 percent of Biden's supporters said they backed Sanders as their second choice, compared to 20 percent who chose Warren as their next favorite. Harris was named as the second option by 14 percent, according to the poll's latest figures that were updated last week.

The survey comes as the Democratic primary heats up, though signs are emerging that the race is becoming a four-way race between Biden, Sanders, Warren and Harris.

The Hill-HarrisX survey was conducted among 138 Democratic and Democratic-leaning Biden supporters between Aug. 16 and 17 with a margin of error of plus or minus 8 percentage points.

—Tess Bonn