Outgoing 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 (D-Texas) narrowly leads former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE as a potential 2020 contender among members of the progressive group MoveOn.org.

A straw poll released by the group on Tuesday put O'Rourke ahead of Biden by little more than half a percentage point — 15.6 percent to 14.95 percent. Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.) comes in after that at 13.15 percent.

A plurality of members surveyed — 17.89 percent — said they either didn't know who they would choose in 2020 or that they preferred "someone else," the survey found.

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The straw poll results listed 33 potential Democratic hopefuls, although one of the would-be candidates on the survey, former Massachusetts Gov. Deval Patrick, has said he will not seek the party's presidential nomination in 2020.

The results bolster O'Rourke's growing clout in the field of would-be Democratic challengers to President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE, especially among progressive and liberal voters, who are seen as crucial to winning the party's nomination.

The soon-to-be-former Texas representative emerged as a sort of rock star in Democratic politics this year as he mounted a high-profile — though ultimately unsuccessful — Senate bid against Sen. Ted Cruz Rafael (Ted) Edward CruzCruz blocks amended resolution honoring Ginsburg over language about her dying wish Trump argues full Supreme Court needed to settle potential election disputes Press: Notorious RBG vs Notorious GOP MORE (R-Texas).

While he threw cold water on the possibility of a White House run earlier this year, O'Rourke has since opened the door to a bid, saying late last month that he and his wife would not "rule anything out."

While O'Rourke narrowly leads the field of potential Democratic contenders listed in the straw poll, MoveOn's members appear to be splintered in their support.

Only four possible candidates — O'Rourke, Biden, Sanders and Sen. Kamala Harris Kamala HarrisHarris faces pivotal moment with Supreme Court battle Nearly 40 Democratic senators call for climate change questions in debates Joe Biden has long forgotten North Carolina: Today's visit is too late MORE (D-Calif.) — managed to get more than 10 percent support in the survey. Sen. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (D-Mass.), who is already building up her operation for a potential 2020 bid, came in fifth place, after Harris, with 6.42 percent support.

MoveOn endorsed Sanders in 2016 after he garnered the support of 78 percent of voters in the group's straw poll to beat out Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE, the eventual Democratic nominee.

But unlike in 2016, the Democratic field is expected to be far more crowded this year, with around three dozen potential candidates rumored to be weighing presidential bids.

MoveOn isn't yet endorsing a potential presidential candidate. That won't come until later, when the group holds a formal vote by its members, the group said in an email.

MoveOn's latest straw poll offered some clues about what progressive voters may be looking for in their next presidential candidate.

More than 63 percent of respondents said they want candidates "who inspire the public with deep progressive values and will move us toward a more progressive future where everyone can thrive," the group said.

Another 54 percent said they are looking for candidates "who make the connections between racial, social, and economic injustice and commit to platform positions to see these connections through," according to MoveOn.