Washington (CNN) 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 ."

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