, the powerhouse grassroots organization that showered Democrats with more donations in the midterms than almost any other liberal PAC, is asking its members whether to host a virtual vote on Thursday to endorse Barack Obama or Hillary Clinton for President.

Spurred by John Edwards’s withdrawal from the race on Wednesday, MoveOn surveyed a sample of its members to gauge endorsement interest, according to a source with knowledge of the group’s operations. Then MoveOn set a deadline of 11 am Thursday for members to back a virtual endorsement vote. If a majority support the idea, virtual balloting will run overnight, open only to the group’s 3.2 million activists, and an endorsement could be announced by Friday.

MoveOn has never endorsed a candidate for President. Last cycle, it required a 50 percent threshold for its presidential endorsement, and Howard Dean fell 6 points short. But now MoveOn has raised the bar to 66 percent– a supermajority that will be hard for either candidate to meet. MoveOn members were largely split between Obama, Edwards, Kucinich and Clinton during its three virtual town halls about public policy last year.

Yet if MoveOn does manage to unite “as a progressive community around one of these candidates,” as Executive Director Eli Pariser explains in a new e-mail, its activists could play a pivotal role in this race. There are over a million and half MoveOn voters in Super Tuesday states. The group boasts 575,000 web activists in California alone–about 9 percent of turnout for the state’s 2004 presidential primary. It has one of the largest and most active donor lists in American politics, which could help finance a long delegate hunt for either candidate. And among many Democratic activists and primary voters, it offers a credential that both candidates covet: a commitment to aggressive, forward-looking politics that will end the war and confront the corrupt GOP establishment. In short, MoveOn is a political “brand” that could blunt lingering Democratic concerns that Obama’s post-partisanship is too nice for the inevitable battles ahead, or validate Clinton’s argument that she is the toughest, most partisan Democrat in the race.

MoveOn’s endorsement dilemma also comes at a critical time for the organization, which was founded about a decade ago to defend Bill Clinton against impeachment. The group has achieved tremendous success in growing its ranks, raising money for Democrats and picking high-profile fights with the Bush Administration. Yet like many progressive groups, it has a strained relationship with Democratic leaders who have failed to end the war, restore constitutional rights or advance a progressive domestic agenda in Congress. If MoveOn–from its battle-tested leadership to its diverse, committed membership–cannot take a clear side in what is now a two-person contest between the Democratic establishment and Democratic reform, then it will have a harder time blaming future politicians for maintaining the status quo.