Single-payer health care appears to be experiencing a surge in popularity among Democrats in Congress.

During the last two years of Barack Obama's presidency, Rep. John Conyers (D-MI) could only find 62 other House Democrats willing to co-sponsor his single-payer health care proposal — which would expand Medicare to cover every American.

But just two months into the new Congress, Conyers's team has already signed up 78 co-sponsors for the exact same single-payer bill. More are expected to come on board in the next two weeks. At this point in the last Congress, only 48 Democratic House members had signed on to the bill.

"During Obama's term, Democrats were uncomfortable with anything that might look like something other than full-throated support of the Affordable Care Act, and they didn't want to do anything that might undermine the president," said Dan Riffle, Conyers's senior legislative assistant, in an interview. "But many members who weren't on the bill, who have had their phones ringing off the hook, are now expressing interest. It's percolating from the ground up."

Several House Democrats see the same thing happening. "There's more of an appetite for an alternative now," Rep. Ro Khanna (D-CA), a sponsor of Conyers's bill, told me. "Democrats have a new confidence to push for a single-payer system. The momentum is building."

Added Rep. Joaquín Castro (D-TX) in an interview: "There is a discussion now that didn't exist a few years ago about how to achieve universal coverage for people."

Of course, Conyers’s single-payer Medicare for All bill is dead on arrival with the Republicans who currently control Congress. But left-wing activists and progressives on the Hill say they’ve made getting the Democratic Party to support a single-payer health care system one of their key priorities — both because they believe it will help the party present a more persuasive alternative to Republicans in the next election, and to lay the legislative groundwork for what they'll enact once they retake the majority.

Since Obamacare’s passage, congressional Democrats have focused on defending Obamacare from the GOP’s ongoing assault. But with Republicans’ health care overhaul collapsing last week, and with Obama leaving office, Democrats are freer than they’ve been in years to pursue a dramatically new direction. The party’s progressive wing is trying to seize on single-payer as that solution.

Some Senate Democrats resist backing single-payer

After Speaker Paul Ryan’s House health care bill imploded, Sen. Bernie Sanders (I-VT) announced over the weekend he’d be launching a new Medicare for All initiative. Like Conyers’s bill, it will suggest allowing every American to enroll in Medicare — a massive new entitlement program likely to be paid for by a new payroll tax, mirroring a similar push Sanders made during the presidential primary. (Read Vox’s Sarah Kliff for a policy explainer on how a single-payer system could work.)

An aide to Sanders said that his bill wouldn’t be released for weeks, but that the senator’s staff is receiving surprisingly positive feedback from other Democratic senators. “We’re seeing much more interest now in the Medicare for All legislation,” an aide to Sanders’s office said. “I think people are looking for bolder approaches now.”

But while Sanders and progressive Democrats clamor for a more aggressive approach, nine Senate Democrats in separate interviews expressed skepticism about the need to go that far, that quickly.

Some pointed to the Affordable Care Act as a better model for Democratic health care policy. “[The ACA] is a more centrist approach. And it gives us a better chance for a broader coalition in support. That’s why President Obama chose that course,” Sen. Ben Cardin (D-MD), a Democratic moderate, told me.

Cardin added that he wasn’t persuaded by left-wing criticisms that single-payer was necessary in part because Obamacare hadn’t done enough to bring down the number of Americans who were uninsured.

“Even if you do single-payer, you’re never going to get to 100 percent,” Cardin said, speaking of uninsured rate, which is currently at a record low of below 9 percent. “There’s gonna be qualifications for undocumented to get covered, people who are in transition and in and out. So you’ll always run to some degree of uninsured.”

The hesitation about single-payer is particularly clear at the party’s leadership level. Asked at the Capitol on Tuesday, Senate Minority Leader Chuck Schumer (D-NY) wouldn’t say one way or another if he thought the Democratic Party should embrace single-payer, only noting that he would review Sanders’s legislation once it was ready. In the House, neither Minority Leader Nancy Pelosi nor Minority Whip Steny Hoyer has officially co-sponsored Conyers’s bill, though Pelosi did tell the Washington Post’s David Weigel that she’s supported single-payer “since before you were born.”

Another prominent Democrat, Sen. Cory Booker (D-NJ), also wouldn’t get behind a single-payer health care system, instead calling it “one of those options that must be considered” in an email to Vox. Sen. Claire McCaskill (D-MO) was openly critical, arguing “it’s important that we keep options open for people who rely on health care.” (Some single-payer plans would get rid of private insurance in favor of government-run care almost entirely.)

Most preferred to duck the question altogether, and concentrate on defending Americans who are covered under Obamacare, as Sen. Bob Casey (D-PA) put it.

“Eighty-five percent of the conversation on health care will be about Donald Trump’s push to repeal the bill,” added Sen. Chris Murphy (D-CT), one of the caucus’s most progressive members, in an interview. “To the extent that there’s not unanimity in the Democratic caucus what to do theoretically — five or 10 years down the line — is not nearly as important as what Donald Trump is going to do to the health care system right now.”

Meanwhile, only a handful of Democratic senators — including Sens. Brian Schatz (D-HI) and Elizabeth Warren (D-MA) — have publicly embraced single-payer. In an interview, progressive Sen. Richard Blumenthal (D-CT) also called himself a “supporter of single-payer” — though, he hastened to add, “I can’t speak for the caucus.”

Why left-wing activists are pushing for a DOA health care bill

The ambivalence and ambiguity of Democrats’ attitude toward single-payer is exactly what some left-wing advocates want to bring to an end. Over the past week, a progressive advocacy group called Justice Democrats, formed after the election, has called the offices of all 119 House Democrats who have not signed on to Conyers’s bill and demanded they do so.

Single-payer enjoys popular support. Gallup found last year that close to 60 percent of the public wants a federally run health care system, and Kaiser polling this March shows more than 66 percent support single-payer. Among Democrats, that number is closer to 81 percent.

“This is Democrats expressing their will in one direction and then party leaders and incumbents going in a different direction. We want to highlight that. The idea is to move these people,” says Corbin Trent, communications director for Justice Democrats, in an interview.

On social media, Justice Democrats have been blasting out a list with the names of every House Democrat who hasn’t signed on to Conyers’s bill. The group is printing 5,000 petitions to deliver to the Washington offices of House Democrats opposing the bill. And they say they’re prepared to back primary candidates against Democratic House and Senate members who do not fall in line behind single-payer.

“We are trying to let them know that the people are watching. And that they want them to fight,” Trent says.

Other left-wing groups are also leading the charge. “The big problem with Obamacare is that it was a Republican idea,” said Neil Sroka, communications director for the left-wing advocacy group Democracy for America. “Having the Democratic Party talk of Medicare for All means it’s doing more than just defending Obamacare. And that’s important because it allows you to recognize the real deficiencies of Obamacare.”

House progressives were also unpersuaded by the argument that the party shouldn’t be articulating a proactive agenda, because they also had to defend the current improvements under Obamacare.

“If Reagan and Goldwater had thought that way, there'd never be a conservative revolution. If Paul Ryan had thought that way, we would never have the conservatism we have today,” Rep. Khanna said.

“Insurance companies have been making a racket of profits over the last couple of years, and the only way to help really contain costs and address Americans’ frustrations is to have Medicare for All.”

But not all of Khanna’s colleagues are willing to go that far. Over the weekend, the Post’s Weigel attended a town hall in Rhode Island at which liberal activists demanded that their representatives push for expanded health coverage.

“We have to look harder at a single-payer system,” Rep. Jim Langevin (D-RI) told them, according to Weigel.

But back at the Capitol, Langevin hadn’t signed on to Conyers’s bill. In a statement, his spokesperson would only say he would “not be cosponsoring” the bill “at this time.” Single-payer, Langevin said in the statement, was just “one of many mechanisms” worth considering to improve American health care.