In a sign that the Democratic Party is embracing more progressive health care ideas, eight Democratic senators announced Thursday that they were co-sponsoring legislation that would allow people 55 and older to buy in to Medicare.

Sen. Debbie Stabenow (D-Mich.) introduced the Medicare at 55 Act with the immediate support of Democratic Sens. Tammy Baldwin (Wis.), Sheldon Whitehouse (R.I.), Sherrod Brown (Ohio), Jeff Merkley (Ore.), Patrick Leahy (Vt.), Jack Reed (R.I.) and Al Franken (Minn.).

The bill, which would allow Americans aged 55 to 64 to purchase Medicare coverage, reflects the growing influence of progressive activists who are pushing for a single-payer health care system they dub “Medicare for all.”

Although the bill stops short of making Medicare universal, its embrace of expanded public health insurance, rather than the private model at the heart of the Affordable Care Act, or Obamacare, marks a distinct leftward shift for the party.

“People between the ages of 55 and 64 often have more health problems and face higher health care costs but aren’t yet eligible for Medicare,” Stabenow said in a statement. “If you live in Michigan, are 58 years old, and are having a hard time finding coverage that works for you, this bill will let you buy into Medicare before you turn 65.”

“Our legislation is one way we can work together on a bipartisan basis to lower health care and prescription drug costs,” she added.

Joshua Roberts / Reuters Sen. Stabenow, who is up for reelection in 2018, introduced the bill on Thursday.

Stabenow’s office suggests the new bill would likely generate cost savings for those between the ages of 55 and 64, since private insurers are permitted to charge that cohort three times the rates of their younger peers due to the generally higher cost of providing them coverage. Americans aged 55 to 64, on average, pay more than $1,200 a year in out-of-pocket medical costs that Medicare would alleviate, according to the Health Care Cost Institute.

The bill would likely also relieve financial strain on Obamacare’s individual insurance exchanges by enabling the departure of some of its most expensive customers. The exchanges are facing mounting problems as insurers withdraw from some areas and increase premiums in others to offset the cost of covering more sick people than they had before Obamacare’s passage.

Stabenow’s bill echoes a similar effort underway in the House of Representatives. In July, Democratic Reps. Jon Larson (Conn.), Brian Higgins (N.Y.) and Joe Courtney (Conn.) announced that they would be introducing the Medicare Buy-In and Health Care Stabilization Act. The bill would permit Americans aged 50 or older to buy in to Medicare for as little as $8,212 a year ― a significant savings for a 60-year-old currently purchasing a high-ranking “gold” plan on the exchange for an estimated $13,308, according to the congressmen’s offices.

The legislation, which they plan to formally introduce in September, would also allot funding and take other measures to stabilize the exchanges. Including Larson, Higgins and Courtney, the bill currently has 15 co-sponsors.

Proponents of a “Medicare for all” system have long argued that the for-profit insurance model has failed, noting that Medicare provides coverage at a significantly lower cost per person. Medicare enjoys widespread popularity, thanks to the coverage it has provided seniors since 1966 (it later expanded to include disability coverage).

Since the collapse of the latest Republican efforts to repeal Obamacare, progressive groups have stepped up pressure on lawmakers and candidates to get behind “Medicare for all.” Sen. Bernie Sanders (I-Vt.) plans to introduce single-payer health care legislation in the Senate in the coming months, and a bill introduced by Rep. John Conyers (D-Mich.) in the House has the support of a majority of the House Democratic Caucus.

But Murshed Zaheed, political director of online progressive organization Credo Mobile, told HuffPost last week that activists welcomed interim steps on the road to single payer.

“We’re not going to pooh-pooh them and say, ‘No, no, we want Medicare for all.’ We will say it is a great starting point,” he said.

Of the Medicare at 55 Act’s co-sponsors, Sens. Stabenow, Baldwin, Brown and Whitehouse are up for re-election in 2018. Stabenow, Baldwin and Brown are running in states that President Donald Trump won in the 2016 election.

The decision by politically vulnerable Democrats to embrace expanded public health insurance speaks to just how much the mainstream Democratic consensus on health care has shifted in the past few years.

As recently as December 2009, talk of lowering the Medicare eligibility age to 55 was enough to endanger the support of then-Sen. Joe Lieberman (I-Conn.) for the Affordable Care Act. And in December 2012, then-President Barack Obama considered raising the Medicare eligibility age in talks with congressional Republicans over a so-called “grand bargain” on the budget.

This story has been updated to include information about a similar bill soon to be introduced in the House of Representatives.