U.S. Sen. Kamala Harris speaks onstage during Children’s Defense Fund – California Hosts 24th Annual Beat The Odds Awards at Book Bindery on Dec. 4, 2014 in Culver City. (Credit: Jesse Grant/Getty Images for Children’s Defense Fund)

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Sen. Kamala Harris is the first Democrat to announce she’ll co-sponsor Sen. Bernie Sanders’ single-payer health care bill when it’s introduced in September.

The California freshman — seen as a rising star in the party and a 2020 presidential prospect — revealed her plans to back Sanders’ legislation at a town hall Wednesday night in Oakland.

“Here, I’ll break some news: I intend to co-sponsor the Medicare-for-all bill, because it’s just the right thing to do,” Harris said. She announced her support for Sanders’ single-payer plan at the end of her town hall and joked that “somebody should tell my staff.”

Her co-sponsorship is another sign that the Democratic Party is increasingly embracing a shift away from the private health insurance market and toward a government-run program. The issue is poised to become a major litmus test for its presidential candidates in 2020.

It comes just 20 months after the party’s last standard-bearer in a presidential election, Hillary Clinton, called Sanders’ single-payer proposal “a theoretical debate about some better idea that will never, ever come to pass.”

“This is about understanding, again, that health care should be a right, not a privilege. And it’s also about being smart,” Harris said. “It is so much better that people have meaningful access to affordable health care at every stage of life, from birth on. Because the alternative is that we as taxpayers otherwise are paying huge amounts of money for them to get their health care in an emergency room. So it’s not only about what is morally and ethically right, it also makes sense from a fiscal standpoint, or if you want to talk about it as a return on investment for taxpayers.”

The early endorsement from Harris could clear the way for more Democrats to co-sponsor the bill. Already this summer, Massachusetts Sen. Elizabeth Warren and New York Sen. Kirsten Gillibrand have announced their support for single-payer health insurance.

Sanders tweeted his thanks to Harris on Wednesday night. “Let’s make health care a right, not a privilege,” he wrote.

Some of Sanders’ most vocal supporters, who had questioned Harris’ progressive bona fides, expressed cautious optimism about Harris’ position.

“Like it or not, single payer has become the litmus test in order to run for office. I expect a a lot or senators to agree. But count on Bernie 100% to actually follow through,” said RoseAnn DeMoro, the executive director of National Nurses United, the first major union to back Sanders’ 2016 presidential campaign.

But DeMoro added she wants to hear single-payer supporters aggressively argue for the end of private insurance, too. “I want to hear an actual articulation of the principles of single payer from these candidates, including that the insurance industry has no role. Bernie says that. Will they?” she said.

Winnie Wong, the co-founder of People for Bernie, said Harris is “meeting her constituents where they are. I see this as a pragmatic political decision.” She added that it could revitalize a state-level debate in California about moving to a single-payer program.