Sen. Kamala Harris (D-Calif.) on Wednesday became the first senator to announce that she would co-sponsor legislation adopting a national, single-payer health insurance system that Sen. Bernie Sanders (I-Vt.) plans to introduce in the coming weeks.

Speaking at a town hall in Oakland, California, Harris called single payer “the right thing to do,” drawing sustained applause from the crowd of hundreds that had gathered to hear her speak.

The junior senator’s revelation is a win for left-leaning activists in two key ways. It adds high-profile support to the “Medicare for all” proposal that has become a top priority for restive progressives. And in allying with the activist base of the Democratic Party, Harris, often shortlisted for the party’s 2020 presidential nomination, is signaling her desire to assuage the concerns of her skeptics on the left.

“This is about understanding, again, that health care should be a right, not a privilege. And it’s also about being smart,” Harris said, explaining her stance at the town hall after the applause died down. “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.”

Gary Cameron / Reuters Sen. Kamala Harris (D-Calif.) called single-payer health care the "right thing to do" on Wednesday.

Harris had previously suggested she viewed single payer favorably, but did not commit to specific single payer legislation. At a July rally, Harris said she was supportive of single payer “as a concept,” but that “we’ve got to work out the details, and the details matter.” She made similar comments on the Pod Save America podcast in May.

Single-payer health care, in which the federal government insures all Americans through one, Medicare-type program, garnered major national attention thanks to Sanders’ 2016 presidential bid, where it was one of his chief policy promises.

At the time, the proposal drew mockery from more mainstream figures in the Democratic Party.

Hillary Clinton, who went on to defeat Sanders for the nomination, famously declared in January 2016 that single payer would “never, ever come to pass.”

But with the election of President Donald Trump and the collapse of Republican efforts to repeal the Affordable Care Act, or Obamacare, which combines private and public health insurance, the push for single payer has gained steam. The framework has picked up the verbal support of Democratic Sens. Elizabeth Warren (Mass.), Kirsten Gillibrand (N.Y.) and Jeff Merkley (Ore.) (None of them has yet to officially co-sponsor Sanders’ bill). In the House, a bill introduced by Rep. John Conyers (D-Mich.) enjoys the backing of a majority of the Democratic caucus ― the most support it has ever received.

Progressive advocates of single payer have long argued that a for-profit health insurance system is doomed to failure, because of how difficult it is to earn a profit without depriving the costliest patients essential care or providing it only at exorbitant prices. They have seized on the struggles of the Obamacare exchanges, which offer private options, and the relative success and political resilience of its more progressive Medicaid expansion as evidence backing their views.

Here's video of the moment Harris announced it. "Here: I'll break some news..." pic.twitter.com/mGbJO6okQE — Daniel Marans (@danielmarans) August 31, 2017

For Harris, the announcement shored up her progressive credentials weeks after coming under increasing scrutiny from activists.

Some supporters of Sen. Sanders in particular expressed concern in July that party leaders and donors had begun grooming Harris for the national stage to circumvent more independent figures that the grassroots might be more inclined to rally behind. Critics pointed to Harris’ decision, during her tenure as California’s attorney general, not to prosecute Steve Mnuchin’s bank for illegal foreclosures against the advice of her department’s Consumer Law Section.

And at the same July rally where Harris said she backed single payer as a “concept,” the senator declined to weigh in on a fight in the California legislature over a state-level single-payer bill. Single-payer legislation backed by the influential National Nurses United labor union passed the California state senate in June. But later that month, California state Assembly Speaker Anthony Rendon (D) refused to bring the bill to a vote in the lower house, sparking the ire of activists.

Winnie Wong, a co-founder of the People for Bernie, who has cast aspersions on Harris in the past, applauded Harris’ support for Sanders’ legislation ― and expressed hope that it would advance the state-level bill in California as well.

“Not all Democrats embrace it, but savvy democrats understand that single-payer agenda is THE topline issue leading into the midterms,” Wong wrote in an e-mail message. “So It’s great to see the JUNIOR Senator from California meeting her constituents where THEY are.”

Virtually all other developed nations enjoy some form of universal coverage, and a single, government insurance program is among the most common ways they achieve it.

Still, implementing such a system is likely to be politically difficult in the United States since it would, among other disruptions, eventually force the 49 percent of Americans who receive insurance through their job to abandon it and join an expanded government plan. It would also require major upfront tax increases to fund this expansion in exchange for the promise of a reduction in overall costs.

In the near term, Sanders’ legislation could expand the bounds of the health care debate, making other progressive proposals seem less radical by comparison. Earlier this month, Sen. Debbie Stabenow (D-Mich.), who is up for re-election in 2018, introduced a bill that would enable people aged 55 or older to buy into Medicare. Last week, Sen. Brian Schatz (D-Hawaii) announced plans to introduce legislation allowing all Americans to buy into Medicaid.

Republicans are already trying to use the single-payer push to brand Democrats as proponents of a government health care takeover paid for by a massive tax hike. The Republican National Committee plans to tie Sen. Bill Nelson (D-Fla.), and nine other vulnerable senators up for re-election in 2018, to single-payer legislation with digital ads that highlight the funding he has accepted from Sen. Warren.

Republican National Committee The Republican National Committee intends to make Sen. Bill Nelson (D-Fla.) pay a price for his colleagues' support of single payer.

Polling suggests there is broad popular support for expanding Medicare, though it appears to depend on how the policy is framed.

Sixty percent of American voters support “an expansion of Medicare that would make it available to any American who wants it,” according to a Quinnipiac University poll published on June 28.

Sanders insists that he is not interested in making support for single payer a “litmus test” for Democrats.

“Is this a litmus test? No, you have to look at where candidates are on many issues,” Sanders told the Washington Post. “But you’re seeing more and more movement toward ‘Medicare for All.’ When the people are saying we need health care for everyone, as more and more Americans come on board, it will become politically possible.”