More than a dozen Democratic senators — including several party power players — stood with independent Vermont Sen. Bernie Sanders on Wednesday as co-sponsors of his latest bill for a government-run, single-payer health care system. The rising support among Democrats for Sanders’ sweeping proposal, which would entail a massive overhaul of a major part of the U.S. economy, marks a clear sea change to the left for Democrats.

“The growing momentum for Medicare for all is a remarkable turnaround for an idea that was deemed too radical to even debate eight years ago. However, it’s really a testament to the political clarity of the policy and the steadfast work Sen. Sanders has put into organizing support for it inside and outside the halls of power,” Charles Chamberlain, a progressive activist and the executive director of Democracy for America, wrote in a statement Tuesday.

As of Wednesday morning, 16 Democratic senators had signed onto the bill, including several more moderate members of the caucus and a number of possible 2020 presidential contenders. A similar bill introduced in the House by Rep. John Conyers, D-Mich., has 117 co-sponsors.

Sanders spokesperson Josh Miller-Lewis told ABC News, “Clearly Democrats are seeing that the vast majority of their constituents and, increasingly, the majority of the American people support single-payer.”

The bill introduced by Sanders today would phase in a universal, government-run health care program over four years. Children up to 18 would be enrolled in Medicare right away, and the minimum eligibility age for the program, which is currently 65 for most people, would decrease over the next few years. By the third year, the Medicare minimum eligibility age would be 35.

“By the fourth year, every individual who is a resident of the United States will be entitled to benefits for comprehensive health care services and will get a Universal Medicare card that they can use to receive the health care they need,” according to Sanders' memo on the bill.

Sanders has been advocating for what he calls a “Medicare for all” health care system for decades, but not one of his Senate colleagues was previously willing to back his legislation. When he made the idea a central part of his presidential campaign, several Democrats, including congressional leaders and the party’s eventual nominee, Hillary Clinton, said the proposal was unrealistic and would be too costly and disruptive to the economy. They accused Sanders of being disloyal to then-President Obama’s Affordable Care Act, which has been under attack by Republicans since its introduction.

“The last thing we need is to throw our country into a contentious debate about health care again. We are not England. We are not France,” Clinton said during a presidential debate in February 2016, arguing against Sanders’ plan. “Based on every analysis that I can find by people who are sympathetic to the goal, the numbers don’t add up, and many people will actually be worse off than they are right now.”

Still, Sanders and progressive lawmakers at the local level and in the House have continued to mobilize grass-roots support around the issue after he lost the nomination and Democrats lost the White House. They argue that health care costs remain too high and that Obamacare does not guarantee universal health care coverage. They pushed the Democratic Party to embrace a vision for more socialized health care even as Republicans were voting to dismantle the Affordable Care Act and put in place market-driven changes instead.

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“Despite the lunatics in the Republican Party, they are not entirely wrong about the [ACA]. It does have serious holes in it,” said Chuck Idelson, the communications director for National Nurses United, a major nurses’ union that was one the first and most committed groups backing Sanders’ bid for the White House.

Idelson added, “Democrats have been recognizing that they lost the last election because they failed to speak to issues that affect people’s daily lives. Nothing affects people’s lives more than their health care.”

A number of progressive political organizations, including the National Nurses United, have effectively made the issue a litmus test for any Democratic candidates seeking their support.

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