Former Vice President Joe Biden Joe BidenOmar fires back at Trump over rally remarks: 'This is my country' Trump mocks Biden appearance, mask use ahead of first debate Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE and Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) are the most popular potential Democratic presidential candidates, but they are also the top second choices among registered voters, according to a Morning Consult survey released this week.

The poll found that Sanders was the second choice for 27 percent of Biden's supporters. Another 15 percent of Biden supporters said they would back Sen. Kamala Harris Kamala HarrisButtigieg stands in as Pence for Harris's debate practice First presidential debate to cover coronavirus, Supreme Court Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (D-Calif.) while 9 percent said they supported Sen. Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE (D-Mass.).

Among Sanders's supporters, 26 percent said they supported Biden as a second choice, followed by Warren at 16 percent and Harris at 7 percent.

Warren and Sanders both hail from the progressive wing of the Democratic Party, while the poll showed some crossover between those supporting Biden and Harris.

Among Harris's supporters, 20 percent said they would choose Biden as a second choice, while Sanders was not among the top picks as a backup option. And among Biden supporters, 15 percent said they would support Harris as a second choice.

However, Sanders registered strongly as a second choice for Warren's supporters, with 25 percent saying the Vermont senator and 2016 runner-up for the Democratic nomination was a backup for them, followed by Harris at 14 percent and Biden at 13 percent.

Among former Rep. Beto O'Rourke Beto O'RourkeJimmy Carter says his son smoked pot with Willie Nelson on White House roof O'Rourke endorses Kennedy for Senate: 'A champion for the values we're most proud of' 2020 Democrats do convention Zoom call MORE's (D-Texas) supporters, 20 percent said they would pick Sanders as a second option, while 18 percent chose Biden.

O'Rourke has yet to announce whether he will run for president next year, though has been mulling a bid.

The poll comes as speculation around potential 2020 runs from Biden and Sanders heats up.

The Hill reported on Thursday that sources familiar with the former vice president's plans say that he is almost certain to enter the 2020 fray.

Yahoo News reported late last month that Sanders would announce his second presidential bid in the near future.

Warren formally launched her White House bid last weekend, while Harris entered the race late last month.

The Morning Consult poll was conducted from Feb. 4-10 among 11,627 registered voters. The survey's margin of error is plus or minus 1 percent.

— Julia Manchester