 -- 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.

“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.

Still, plenty of Democrats are opposed to the bill or at least remain skeptical of the concept. Tom Perez, the chairman of the Democratic National Committee, hedged Tuesday when asked about the growing momentum around Sanders’ bill. He said it was one of many ideas among members of the party for achieving universal health care coverage in the country but cautioned that Democrats are still playing quite a lot of “defensive” on the issue.

“We have always believed in universal health care. We are 90 percent of the way there. We are fighting to get the final 10 percent and fighting against a Republican administration that wants to take away a big part of the 90 percent,” Perez told ABC News on Tuesday. “The question is, what is the fastest way to get to 100 percent? ... Different Democrats may have different pathways to get there, and that is the debate that will ensue.”

Idelson speculated that it was easier for Democrats to get on board with the idea of single-payer now, with the GOP controlling Washington, since they do not have much chance of passing or implementing such legislation. Many Democrats, perhaps, have calculated it is better to avoid alienating activists on the left who have been incredibly animated since Donald Trump took office. “But we are going to remind people of [their co-sponsorship]. We are going to remind people that this is really the only solution to the health care crisis,” Idelson added.

Sanders’ team has acknowledged that a Republican-controlled Congress and GOP administration will almost certainly never consider such legislation. Lewis-Miller said the bill was designed as a negotiating tool.

“The bill we are introducing is not going to be the final bill that is signed into law, but it is going to be the beginning of a conversation about what single-payer in the United States would look like,’ Miller-Lewis said.

He was delighted that the proposal has broken through into the mainstream and will be a point of central consideration and debate in Democratic politics.

“[Kirsten] Gillibrand, [Cory] Booker, [Elizabeth] Warren, Kamala Harris, Bernie Sanders — is there anyone else who is possibly running for president in 2020?” he said, listing some of the heavy hitters in the Senate backing the bill. “It’s amazing … That now will mean that the conversation about single-payer Medicare for all is at the heart of the Democratic Party.”