When Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.) last introduced a single-payer bill in 2013, it didn’t attract a single co-sponsor. Now, as he unveils his “Medicare for all” bill on Wednesday, some of the biggest names in the Democratic Party will be by his side.

It’s a vindicating moment for Sanders, who is seen as a leading contender for the Democratic nomination in 2020.

The Vermont senator’s bill has virtually no chance of passing this Congress, and many Democrats, including members of leadership, remain wary of the idea.

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But that doesn’t detract from the scope of his accomplishment. From the start of his insurgent presidential campaign last cycle, Sanders’s goal was to drive Democrats into his camp on health care and other issues — and it’s working, perhaps better than he could have ever imagined.

“I guess this is why the 2016 Democratic primary was a terrific thing,” said Jonathan Tasini, a prominent progressive organizer and former Sanders campaign surrogate.

“Without that primary, ‘Medicare for all’ — the idea that millions of people would have what everyone has around the world — would not be in the conversation.”

In the past week, several other potential 2020 contenders, including Democratic Sens. Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE (Mass.) and Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (N.J.), have embraced Sanders’s bill by signing on as co-sponsors. And it’s not just liberals who are warming to the idea.

Former Sen. Max Baucus Max Sieben BaucusBottom line Bottom line The Hill's Morning Report - Presented by Facebook - George Floyd's death sparks protests, National Guard activation MORE (D-Mont.), one of the architects of ObamaCare, had long been critical of single-payer health care. But last week, he said lawmakers should start looking at the idea.

Just two years ago, Sanders formally announced he was running for president in a sparsely attended press conference that lasted 10 minutes. At the time, critics derided him as a socialist laughingstock; today, he’s one of the most popular politicians in America, giving him a megaphone to promote his single-payer bill.

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“This week will be seen as a pivotal moment, when the history books are written, on ‘Medicare for all,’ ” said Adam Green, co-founder of the Progressive Change Campaign Committee.

Sanders made “Medicare for all” a central part of his platform in the 2016 race. That led to clashes with eventual Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE, who favored incremental tweaks to the Affordable Care Act (ACA) and derided his ideas as unrealistic.

“I think the big thing that Bernie Sanders showed during the 2016 race was the hunger across the ideological spectrum for big, bold solutions to the problems our country faces, not the least of which is health care,” said Neil Sroka, spokesman for the liberal group Democracy for America.

Those supportive of “Medicare for all” think the momentum is on their side. They say it’s only a matter of time before every Democrat elected or running for office will have to take a position on single-payer health care.

“What’s different about this moment is this is no longer going to be a fringe position,” said Robert Weissman, president of Public Citizen, a liberal advocacy group in D.C. founded by Ralph Nader.

“Elected officials are going to have to take a position on ‘Medicare for all’ on the merits. They’re not going to be able to say anymore, ‘That’s not a serious policy position.’ Now it is a serious policy position.”

It remains to be seen whether the single-payer legislation will help or hurt Democrats, particularly in states where they lost ground to President Trump in the election.

Notably, Senate Democrats up for reelection in states won by Trump last year are mostly treating Sanders’s bill with caution.

Some have said they want to build upon and improve ObamaCare. Others have said they support adding a government-run health plan to compete beside private plans or adding a Medicare buy-in for adults 55 and older. In the past, critics have pointed to just how costly a single-payer insurance system would be.

“I think that particular proposal is premature,” said Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (Mo.), a vulnerable Democrat up for reelection in 2018, about Sanders’s plan.

Sen. Joe Manchin Joseph (Joe) ManchinMomentum growing among Republicans for Supreme Court vote before Election Day Gardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' Barrett seen as a front-runner for Trump Supreme Court pick MORE (D-W.Va.), also up for reelection next year in a state where Trump is popular, said he’s skeptical that single-payer can work.

“Let it go through the committee, let it go through the process. I don’t just know enough about it. I’m not signing on to a piece of legislation that I don’t have any idea what it’s going to do to the economy, to the access and to people’s care,” he told The Hill.

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The debate erupting in the Democratic Party over single-payer comes at a time when the future of ObamaCare remains uncertain, despite the GOP’s failed effort to repeal and replace the law.

The Trump administration has slashed funding to enroll people in ObamaCare this fall. Meanwhile, the marketplaces for coverage remain on shaky ground, with insurers pleading for certainty that may never come.

While Democrats are still defending ObamaCare, many are moving on to the next fight.

Sanders’s popularity with the liberal grass roots has made it difficult to imagine Democrats selecting a nominee in 2020 who doesn’t back single-payer; still, some progressive groups and Sanders insist it shouldn’t be a litmus test.

“In 2020, it’s highly probable that the Democratic nominee is actively campaigning on ‘Medicare for all,’ not just endorsing it,” said Green with the Progressive Change Campaign Committee.

“This is a steep change from where the Democratic Party was in 2009 and even where the center of gravity was a few months ago.”

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Sroka said Sanders has “been smart about building a coalition” in support of his single-payer plan.

“Statements of support you’re hearing today weren’t just gotten by asking them this week, right? That has been a building process.

“It takes a lot of work, and lot of time, and a lot of effort to build that kind of momentum behind a bold idea like this.”

Peter Sullivan, Nathaniel Weixel and Ben Kamisar contributed.