An early straw poll of members of the progressive group MoveOn.org shows a wide-open competition for liberal voters in the 2020 Democratic presidential contest, with Rep. Beto O'Rourke narrowly beating out former Vice President Joe Biden.

The poll, obtained by NBC News, shows a plurality of respondents — 29 percent — either said they did not yet know whom they would support or wanted someone else not listed among the group's more than 30 potential candidate choices.

The most popular potential candidate was O’Rourke, D-Texas, who was selected by 15.6 percent of respondents, followed by Biden at 14.9 percent, and then Sen. Bernie Sanders, I-Vt., with 13.1 percent.

It's another sign of O’Rourke's surprising popularity among national Democrats and a potentially troubling indication for Sanders, whom MoveOn endorsed in the 2016 Democratic primary. That year, 78 percent of MoveOn members voted to back Sanders over Hillary Clinton

The three men were followed by Sen. Kamala Harris, D-Calif., who garnered 10 percent support, and Sen. Elizabeth Warren, D-Mass., with 6.4 percent. Meanwhile, three Democratic senators, Sherrod Brown of Ohio, Amy Klobuchar of Minnesota and Cory Booker of New Jersey, and former New York City Mayor Michael Bloomberg were each selected by about 3 percent of members.

"While the race for the 2020 Democratic nomination for president remains wide open and MoveOn's endorsement is up for grabs, MoveOn members and progressives across the country are clear: They're looking for candidates who will rally voters around a progressive vision of building a country where every American can thrive — whether we're white, black, or brown, rich or poor," said Ilya Sheyman, executive director of MoveOn Political Action.

"We'll be challenging prospective candidates to inspire us with big ideas in the months to come — including at a series of events in early voting states in early 2019," Sheyman added.

MoveOn, which was founded during Bill Clinton's presidency, is one of the largest progressive online organizing groups with millions of members across the country, so its endorsement has been coveted in the past.

This year, the group plans to hold a series of events in early presidential nominating states as part of its endorsement process, which is ultimately decided by a vote of its members.

Here are the top 10 finishers in the MoveOn straw poll:

Someone else/DK/other: 28.8 percent

Beto O’Rourke: 15.6 percent

Joe Biden: 14.9 percent

Bernie Sanders: 13.1 percent

Kamala Harris: 10 percent

Elizabeth Warren: 6.4 percent

Sherrod Brown: 2.9 percent

Amy Klobuchar: 2.8 percent

Michael Bloomberg: 2.7 percent

Cory Booker: 2.6 percent