WASHINGTON ― Democratic leaders in Congress are offering to cooperate with Republicans on solutions to stabilize the Obamacare exchanges, but progressive groups and their congressional allies are heading in the opposite direction.

Following the defeat of Republican efforts to repeal the Affordable Care Act, or Obamacare, left-leaning organizations are accelerating their long-standing goal of establishing a single-payer system in which the government provides health insurance for all.

Although proponents of what they call “Medicare for all” acknowledge that such a radical overhaul of the health care system is impossible as long as Republicans control the White House and Congress, they want it to become a litmus test for 2020 presidential candidates ― and a winning issue for as many 2018 congressional candidates as possible.

That way, the thinking goes, when Democrats return to power, they will recognize that single payer, or a number of interim steps favored by advocates, is the only politically viable health care policy.

“The Democratic position has to be guaranteed health care with single-payer financing,” said Michael Lighty, director of public policy for National Nurses United, a 150,000-member labor union that has been at the forefront of single-payer advocacy. “The activists who volunteer for campaigns and put their heart and soul into the party, aren’t going to be satisfied with anything less than Medicare for all.”

“All Democrats who have any aspiration to run for national office must support Medicare for all,” said Murshed Zaheed, political director of Credo Mobile, a phone company that channels its revenue into progressive online organizing. “That automatically puts certain people ― Cory Booker, Kamala Harris and Kirsten Gillibrand, for example ― on the spot: Are they going to support Medicare for all or not?”

On Friday, Credo launched an email campaign asking activists to sign a petition telling “all Democrats running for national office: Support Medicare for All.” The group plans to gradually send the appeal to its entire email list of more than 5.7 million people.

The goal, Zaheed said, is to make opposition to single payer as politically radioactive for 2020 Democratic presidential candidates as support for the Iraq War was in the 2008 primary. In Democrats’ 2008 nominating contest, then-Sen. Barack Obama’s opposition to the war from its outset helped him defeat then-Sen. Hillary Clinton, who had voted to authorize the invasion in 2002.

At the same time, Zaheed said, they welcome more modest proposals that lay the foundation for full, publicly provided coverage, including the creation of a public health insurance option, lowering the Medicare eligibility age and expanding Medicaid.

“We’re not going to pooh-pooh them and say ‘No, no, we want Medicare for all.’ We will say it is a great starting point,” he said.

Bloomberg/Getty Images Rep. Ro Khanna (D-Calif.) recently signed a pledge to "stand up for Medicare for all" whenever publicly discussing health care reform.

Credo’s initiative joins the growing chorus of progressive voices pushing to make single payer an official Democratic stance.

Single payer has more support among congressional Democrats than ever ― on paper, at least. Sen. Bernie Sanders (I-Vt.), whose 2016 presidential campaign shined a national spotlight on single payer, plans to introduce single-payer legislation, though he has yet to announce a date for the bill’s rollout.

A single-payer bill Sanders introduced in 2013 did not attract any co-sponsors. But he is poised to pick up more backers this time around: Democratic Sens. Elizabeth Warren (Mass.), Jeff Merkley (Ore.) and Kirsten Gillibrand (N.Y.) are all on the record in support of “Medicare for all.”

In the House, Rep. John Conyers’ legislation creating a national health insurance program, which he has introduced multiple times in the past, now enjoys the support of a majority of the House Democratic Caucus.

Long before it became clear that Obamacare repeal was headed for defeat, liberal activists set out to press their advantage. Last Tuesday, Our Revolution, National Nurses United, Democracy for America and other like-minded groups announced a campaign to get House Democrats to endorse a slate of progressive bills, including Conyers’ single-payer bill, that they are calling the People’s Platform.

The movement for single payer is backed by a firm belief that the for-profit health insurance model has failed. Virtually all of the world’s other developed nations guarantee health care to their citizens through direct government insurance or something similar, these critics note. Unlike the United States, these countries enjoy universal coverage ― and they provide it at a much lower cost per person than the private-insurance-heavy U.S. system.

What’s more, advocates observe, Medicare, which is effectively a single payer program for seniors and people with disabilities, already does a better job controlling costs than its private insurance counterparts.

Now that Republican efforts to repeal key provisions of Obamacare appear to be dead, progressive activists see an opening to shape health care reform that they believe they were denied when Obamacare was crafted in 2009.

“There’s more optimism on the health care issue, ironically, than there has been since 2008 ― both because of the failure of repeal and the excitement for Medicare for all,” Lighty said.

From the beginning, these progressives have blamed Obamacare’s shortcomings on its reliance on private insurance companies. They are eager to point out that the most troubled part of Obamacare ― coverage on the exchanges ― relies on private insurance, while the law’s expansion of Medicaid, a government insurance program for low-income Americans, has proved far more successful. In fact, thanks in part to the staunch opposition of Republican governors, rolling back Medicaid expansion proved so politically toxic that it was not even included in the so-called skinny Obamacare repeal bill that collapsed on the Senate floor Friday morning.

“Policymakers have tried to expand almost every aspect of the current system in order to increase access,” Lighty said. “The one thing they haven’t done is expand Medicare. We’ve already tried everything that doesn’t work. Let’s try the one thing that’s working.”

We’ve already tried everything that doesn’t work. Let’s try the one thing that’s working. Michael Lighty, National Nurses United

The trouble for activists like Lighty is that many Democrats, eager to curry favor with grassroots activists, have given verbal support for single-payer insurance only to shy away from it when given the chance to actually act on their beliefs.

In California, where Democrats control the governorship and enjoy supermajorities in both chambers of the state legislature, National Nurses United led an effort to advance state-level single-payer legislation. The bill passed the state Senate, but in June, Democratic Assembly Speaker Anthony Rendon refused to allow it to come up for a vote in the lower house. The result infuriated activists because similar legislation had passed both legislative chambers in the past, albeit at a time when a Republican governor ensured that it stood no chance of becoming law.

The experience shows the limits of asking Democrats to simply check a box, according to Lighty. After lambasting Rendon for his decision, NNU has turned to raising awareness of the issue in Assembly districts where incumbent Democrats are more vulnerable. If Democrats in those districts can be convinced to get behind the initiative, the thinking goes, Rendon won’t feel compelled to spare those members a tough vote.

Others groups are testing their own methods of pushing lawmakers beyond paper-thin commitments to single-payer health care. Justice Democrats, a political action committee that is backing progressive primary challengers to 11 congressional Democrats, rolled out a campaign Friday asking Democratic congressional incumbents and candidates to sign the “Stand up for Medicare” pledge. The pledge asks signers to “stand up for Medicare for all in any public appearances and statements addressing our country’s healthcare crisis,” rather than merely affix their name to a piece of legislation.

Thus far, Reps. John Conyers (D-Mich.) and Ro Khanna (D-Calif.) are the only incumbents who have signed the pledge. (Both spoke in support of the initiative after signing it at a news conference near Capitol Hill on Friday.)

Democratic leaders in Congress have taken a decidedly different approach after the collapse of Obamacare repeal.

In a Friday letter to Republican congressional leaders, House Minority Leader Nancy Pelosi (D-Calif.) invited their collaboration on allotting funding to help stabilize Obamacare’s individual insurance exchanges.

For lawmakers concerned about spiraling costs and dwindling coverage options in the exchanges, it’s a natural place to start. As Pelosi noted, Republicans already set aside funds for the purpose in one of their failed repeal-and-replace bills.

A new bipartisan group of about 40 moderate House Democrats and Republicans wants to take it a step further, coupling funding to entice insurers to stay on the exchanges with looser regulation of existing plans.

Pacific Press/Getty Images Progressive activists demonstrate for single-payer health insurance in Manhattan's Union Square on July 2.

But Democratic leaders have declined to make single payer part of “A Better Deal,” the economic agenda they released last week.

Khanna revealed Friday that the Congressional Progressive Caucus had tried to get Senate Minority Leader Chuck Schumer (D-N.Y.) to add single payer to the party’s new agenda during a recent meeting.

“There’s a simplicity in standing for Medicare for all, for free public college that I think will break through the noise,” Khanna said.

Of course, Democrats have reasons to be squeamish. Republicans are already salivating at the prospect of Democrats getting behind single payer, casting it as a massive tax increase. During a series of Obamacare repeal votes Thursday, Sen. Steve Daines (R-Mont.) tried to sucker Democrats with a last-minute amendment that would have expanded Medicare to the entire population. The trick, which fell through when Sanders refused to play along, would have forced vulnerable Senate Democrats to choose between spurning their base and signing onto an ambitious progressive plan that would require tax hikes.

The Republican line of attack should not scare Democrats, according to Lighty.

Indeed, 60 percent of Americans favor “an expansion of Medicare that would make it available to any American who wanted it,” according to a June Quinnipiac University poll.

“I don’t really understand why [Republicans] think that making the most popular government program available to everyone is a political loser,” he said. “When people actually see tax dollars going for things that benefit them and that meet their needs directly, they tend to support those taxes.”