Atlanta (CNN) The activist Ady Barkan announced on Wednesday that he is endorsing Elizabeth Warren for president, a strong sign of progressive support for the Massachusetts senator after her rollout of a "Medicare for All" transition plan last week that received mixed reviews from leading single-payer advocates.

Barkan, who is dying of ALS after being diagnosed in 2016, has emerged over the last few years as one of the leading health care activists in the country, incorporating his own story into a broader appeal for a massive overhaul of the American health care system.

During this Democratic primary campaign, nearly all of the party's leading candidates have made the journey to California, where Barkan lives with his wife and two children, to discuss the issue in a series of taped interviews.

In the past month, as Warren first unveiled her proposal to finance Medicare for All and, more recently, with the release of her transition plan, Barkan has backed her over the concerns of some fellow activists.

"We can and should talk about our strategy and our tactics," he wrote in an op-ed published early Wednesday by The Nation . "But what matters most to me is that Elizabeth Warren is all in for Medicare for All . Her plan says clearly that by the end of her first term everyone will have comprehensive guaranteed Medicare -- whether you are rich or poor, young or old; that there will be no co-pays, premiums or deductibles; and that we will bring down the costs of healthcare because private insurance companies will no longer be able to put profit over patients."

Warren in a statement thanked Barkan for his endorsement.

"I'm deeply grateful to have Ady Barkan's support in this fight," she said. "I'm making a promise to Ady, his family, and every family in America: I will fight for your family as hard as I fight for my own. We're going to do this together."

This past spring, Barkan testified at the first congressional hearings on Medicare for All.

"Some people argue that although Medicare for All is a great idea, we need to move slowly to get there," he said during a now-rare trip to Washington. "But I needed Medicare for All yesterday. Millions of people need it today. The time to pass this law is now."

But Barkan's relationship with Warren traces back further than the current election cycle. In 2012, he launched a pressure campaign on the Federal Reserve, calling for it to prioritize employment and wage growth over concerns about inflation.

"I've worked with Elizabeth since before I was sick," Barkan recalled. "She was a key partner for the Fed Up campaign, an effort I led to demand that the Federal Reserve use monetary policy as a vehicle for good, instead of as a handout to Goldman Sachs."

Warren sat with Barkan earlier this year to discuss the issue, a conversation that pushed Warren deeper into the ideological weeds of the debate than she had previously gone. Asked by Barkan how she ultimately squared her support for Medicare for All -- which would effectively ban private health insurance -- with her defense of well-regulated private markets, Warren offered what progressives considered a notable concession.

"There are areas where markets just don't work and a big part of health care is one of those. So the idea that we could get a couple of regulations in place and it will all sort itself out is just not true with health care," Warren said. "I get financial decisions over whether you're gonna to be able to get a new car or what it looks like, but not financial decisions at the heart of basic health care. Medicare for All, it's about a relationship that all of us as Americans have to each other."

Last week, Warren put out her new plan to transition the country to Medicare for All, first by using the Senate budget reconciliation process to pass legislation that would immediately offer Medicare for All's full suite of benefits -- at no cost -- to children under 18 and people at up to 200% of the poverty level, around $51,000 in income for a family of four. The option would be open to any American who wants to use it, but they would have to pay for it, though costs would decline over time. Additionally, the legislation would lower the current Medicare eligibility age to 50 from 65, while also expanding the program's benefits and lowering prices for enrollees.

But a full push to move to Medicare for All, she said, would only come after the first piece was implemented and had a chance to take hold. Warren has pledged to finish the transition before the end of her first term in office. Amid some criticism suggesting Warren was checking her support for the policy, Barkan defended her.

"Today, @ewarren laid out a plan to get 100 million ppl onto Medicare in first 100 days, via exec action & 51-vote reconciliation," he tweeted last week. "And to use that victory to win midterms & show America that M4A is a great+workable. It's a damn smart plan. And by now, that should surprise no one."

In his piece in The Nation explaining why he would vote for Warren in the California primary, Barkan also makes clear his fond feelings and support for Bernie Sanders , the Vermont independent senator who wrote and popularized the Medicare for All legislation.

"For progressives like me -- and maybe you -- the choice in this primary is between Elizabeth and Bernie. It is a difficult and wonderful choice to have," Barkan wrote. "I believe that either one of them would be the best, most progressive president in modern US history."