Third Way has put together research showing that a third-party campaign would hurt Biden, an argument that No Labels has dismissed. Bennett declined to comment specifically on this month’s Capitol Hill meeting but confirmed that Third Way is working urgently to stop a third-party candidate.

“Very often there are differences of opinion or strategy when it comes to the Democratic coalition, because it’s very, very broad. But here, there’s unanimity, and everyone agrees that if they go forward this is going to hurt Joe Biden,” Bennett said in an interview. “We need to make clear to folks that what they are selling is an illusion, not a choice.”

The alliance between the party’s leading centrists and prominent liberals to publicly squash a third-party effort demonstrates how seriously Democrats fear that a spoiler candidate could tip the election to Donald Trump or another Republican candidate. If next year’s presidential ballot is as close as 2016 or 2020 were, Democrats worry that Trump-weary voters could defect from Biden to an alternative candidate — and just a few thousand of those defections could be decisive in the Electoral College.

Liz Cattaneo, a spokesperson for MoveOn, said that the group is “working with a broad range of Democratic organizations to stop No Labels from running a third-party presidential ticket.” She added that her organization is “committed to accountability for No Labels and to preventing right-wing extremists from winning back” the White House.

No Labels is unbowed. Ryan Clancy, the group’s chief strategist, said that “it shouldn’t surprise anyone … that voters are more open to an independent than ever before. It’s why our polling shows an independent ticket has a viable path” to winning.

Manchin has argued that there’s little harm in his entertaining a third-party bid, and he’s refused to rule one out even as his colleagues try to talk him out of it. Both Democrats and Republicans are “being driven by business extremes” and catering to the “far right and far left,” he said in an interview on the topic last month.

Briefing top Hill Democrats about No Labels is a clear move to get the party on the same page in opposition to the group’s work. And all these dynamics could make the difference between the Senate majority and minority come 2025: If Manchin runs for the White House instead of reelection in West Virginia, Democrats could end up losing both his Senate seat and the presidential race.

A Manchin aide said that if his schedule permits, the senator’s chief of staff will attend the July 27 meeting.

Manchin is the candidate most frequently mentioned as a potential recruit for No Labels, which is eyeing a budget as high as $70 million for its third-party initiative. But there’s also some private talk about former Maryland GOP Gov. Larry Hogan or even Independent Sen. Kyrsten Sinema of Arizona leading an alternative slate, though Hogan says he won’t run for president and Sinema generally stays away from such speculation.

Any of those three candidates on a presidential ballot could influence a close contest for the Electoral College next fall. Dritan Nesho, No Labels’ chief pollster, said that polling “shows an overwhelming opening for a third-party ticket before names are even announced and any campaign communicating the vision and issue positions is run.”

Detractors counter that the group is wildly overstating its chances.

“What we are trying to make clear to the people around No Labels, especially people thinking about running on their ticket, is that that is a preposterous pipe dream. And they can have an impact, but it isn’t by winning,” Bennett said. “They can have an impact by spoiling.”