After years on the fringe, Medicare for All is becoming a central plank of the Democratic Party. Dozens of Congressional Democrats are set to announce the formation of an official Medicare for All Caucus on Thursday, according to a source familiar with the plan.

The caucus is expected to have over 60 founding members, or approximately a third of the 193 House Democrats, with more expected to sign on in the coming weeks. More could join after the November midterms with many Democratic challengers embracing Medicare for All.

It's just the latest instance of Democratic lawmakers embracing a single-payer healthcare system after decades of party leaders dismissing it as too left-wing and politically toxic in the face of Republican attacks.

“This is a sea change from just four or five years ago”

“This is a sea change from just four or five years ago and people are more likely to see healthcare as a right,” said Democratic Rep. Pramila Jayapal of Washington, who will be a caucus co-chair along with Reps. Keith Ellison of Minnesota and Debbie Dingell of Michigan. Members of Congress will formally announce the caucus at 10 a.m. Thursday at the Capitol along with representatives from National Nurses United, Social Security Works, and other progressive groups.

“The impetus for this caucus is to have a real discussion and work with our offices to see how a system would really work as we think about implementing it. It’s about a growing movement to bring a Medicare for All system into being,” Jayapal said, adding that she wants to caucus to work on having proposals ready if the Democrats take back control of the House in 2018.

Medicare for All became a divisive issue during the 2016 presidential primary race between Democratic Socialist Bernie Sanders, who has long advocated for the program, and Hillary Clinton, who dismissed it as too expensive and said it was politically “never, ever” going to happen.

But in the aftermath of Clinton’s defeat, Republicans' attempts to undo Obamacare, and rising healthcare costs, a large swath of Democrats shifted their positions. When Sanders reintroduced his Medicare for All bill last fall, many of the party’s most prominent senators and potential 2020 presidential candidates, including Cory Booker, Elizabeth Warren, and Kirsten Gillibrand, signed on to the bill (in the years before, Sanders had been the lone co-sponsor).

“Bernie Sanders putting that out at that high level was a huge step forward. It built the public knowledge that this wasn’t a crazy progressive idea,” said Jayapal.

The bill lacked many details, especially how to pay for the program, but it brought the idea into the Democratic mainstream. When a Medicare for All bill was introduced in the House of Representatives, 122 Democrats announced they’d co-sponsor it. And on the campaign trail for the 2018 midterms, many Democrats in the most competitive swing districts are also embracing Medicare for All. Republicans have responded with the familiar and previously successful playbook of attacking the Democrats for embracing “socialized medicine” and warning of healthcare rationing.

Clinton, for her part, was skeptical of Democrats embracing the new bill. "I think it’s going to be challenging if within that bill there are tax increases equivalent to what it would take to pay for single-payer, and if you’re really telling people — about half of the country — that they can no longer have the policies they have through their employer," she told Vox last fall.

But many Democrats appear undeterred. Some point to a recent poll by the Kaiser Family Foundation showing that 59 percent support a government health care system for everybody along the lines of Medicaid and Medicare. Progressives also argue that skyrocketing health care costs have made Americans more receptive to ideas that seemed radical in the past.

And it looks like we'll have a chance to find out if they're right. With the sign-on by some of the biggest names in the party, Medicare for All could be a plank of the Democratic Party’s official platform in 2020 and at the center of the debate when the party takes on President Donald Trump.