Sen. Elizabeth Warren on Thursday announced her support for an upcoming bill from Vermont’s independent Sen. Bernie Sanders.

Sanders said in March that he would follow through on his long-held support for single-payer insurance by introducing a bill extending Medicare-like coverage to achieve universal health care. The bill, which is still being crafted, is due to be unveiled Wednesday

Warren, a Democrat from Massachusetts, shared the news with her supporters in an email that began, “I’m co-sponsoring Bernie’s Medicare-for-All bill.”

Warren follows California Sen. Kamala Harris, who recently backed the Sanders bill at an Oakland town hall. The three senators are considered top-tier contenders for the 2020 Democratic presidential nomination, should they decide to run.

Warren’s support of the bill unites the two most powerful members of the party’s left flank, which used to be called the “Warren wing” before being rechristened the “Sanders wing,” given his surprisingly close contest with Hillary Clinton in the 2016 election.

To have Sanders, Warren, and Harris on the same bill sends a signal that this is the central Democratic vehicle for health care policy reform going forward, which stretches the contours of a debate that previously excluded single-payer.

“There’s a lot of potential to really grow this idea if you have six or seven Democrats all talking about why we need a single-payer system,” said Josh Miller-Lewis, a spokesperson for Sanders, on the Warren endorsement. “This is a big moment.” Miller-Lewis added that the bill was still being written.

Signing on to a single-payer bill with momentum may seem like an obvious political move for Warren, but it’s not that simple. Warren has achieved stature in Congress; signing on to a colleague’s bill rather than drafting one to her own specifications is a concession to Sanders. The move is a nod to Sanders’s elevation within the progressive movement.

Indeed, embedded in Warren’s letter to supporters is a note that Sanders’s bill is “one way” to address health care policy. “Medicare for All is one way that we can give every single person in the country access to high quality health care,” she wrote. “Everyone is covered. Nobody goes broke paying a medical bill. Families don’t have to bear the costs of heartbreaking medical disasters on their own.”

Warren has also had a complicated past political relationship with single-payer schemes. During her 2012 campaign for the Senate, her opponent, Republican Scott Brown, criticized her for backing single-payer, while Warren’s leftist primary opponent criticized her for not backing it.

“I made a clear statement I’m the only candidate in this race who supports single-payer,” Marisa DeFranco, Warren’s challenger from the left, told MassLive.com. “No one disabused me of that notion.” Warren never explicitly backed single-payer in that campaign, instead saying that the focus needed to be on defending the Affordable Care Act, Barack Obama’s signature health care reform law.

Brown, however, based his charge of Warren’s support for single-payer on a chapter in a 2008 book she co-authored with Deborah Thorne, a University of Idaho professor. In the book, “Health at Risk: America’s Ailing Health System — and How to Heal It,” edited by Jacob Hacker, who would later become known as the father of public option, they wrote: “We approach the health care debates from a single perspective: maintaining the financial stability of families confronting illness or injury. The most obvious solution would be universal single-payer health care.”

Thorne and Warren, then a professor at Harvard Law School, added that such a solution may be “politically unacceptable.”

It’s not anymore.