(Bloomberg) — Sxteen Senate Democrats are flirting with a single-payer health-care system, marking a shift within the party on what was once viewed as a politically treacherous issue that attracted little support from lawmakers.

The Democrats are co-sponsoring a measure proposed Wednesday by Senator Bernie Sanders, a Vermont independent, that would expand Medicare coverage to all Americans. That’s a significant change from 2013 when Sanders, who lost the 2016 Democratic presidential nomination to Hillary Clinton, attracted no co-sponsors.

Support for the legislation comes after the Republican-controlled Senate fell one vote short in July of adopting a narrow repeal of Obamacare, and as lawmakers express concern that insurance marketplaces could collapse without new government support.

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Despite new support from Democrats for Sanders’s measure, that doesn’t mean it stands a chance of passage. Republican lawmakers, who control both chambers of Congress, oppose a single-payer system.

Several Democrats, including some of Sanders’s co-sponsors, made it clear that they see the bill as one of many options toward improving the Affordable Care Act, President Barack Obama’s signature domestic accomplishment.

“The principle that I support is universal, accessible, affordable quality health care for all, and I think the single-payer system is a strong articulation of the principle,” said Senator Mazie Hirono of Hawaii, a co-sponsor of the bill.

Support for Sanders’s plan may be stronger among potential 2020 presidential candidates seeking to appeal to those who backed the Vermont senator’s campaign last year. Cory Booker of New Jersey, Kamala Harris of California, Elizabeth Warren of Massachusetts and Kirsten Gillibrand of New York –- all frequently mentioned as potential candidates –- are among the co-sponsors.

Even the Senate’s most conservative Democrat, Joe Manchin, of West Virginia, said he’s open to considering a single-payer system, though he said he’s skeptical it’s the right solution. He said he’s not ready to sign onto Sanders’s bill.

Among moderate Democrats, reaction to Sanders’s approach ranges from skepticism to tentative openness. Senator Claire McCaskill of Missouri said the proposal is “premature,” and she would prefer allowing people age 55 to 64 to buy into the Medicare program.

“It’s great to have as many ideas as possible on the table,” said Senator Chris Van Hollen of Maryland, the chairman of the Democratic Senatorial Campaign Committee. “I’m gonna take a look at it myself.”

Progressive groups that have been urging the party to move to the left likely will rally behind Sanders’s proposal.

“The Democratic Party is increasingly wrapping itself in the flag of Medicare for all,” said Adam Green, co-chairman of the Progressive Change Campaign Committee. “Bernie Sanders proved the concept of how popular and motivating this can be for voters.”

Senator Tim Kaine of Virginia, Clinton’s 2016 running mate, said he and Senator Michael Bennet of Colorado, along with a few other senators, have been working for several months on a plan to provide a public health-care option.

“I want everybody to be able to buy a public health plan; I want more choices rather than fewer,” Kaine said.

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