Elizabeth Warren nets endorsement of influential health care activist Ady Barkan

It signals key support as progressive Democrats vie to bear the party standard.

Atlanta -- Ady Barkan, the terminally ill liberal health care activist, announced he's endorsing Sen. Elizabeth Warren, D-Mass., on Wednesday.

Barkan's voice adds a major vote of confidence to her 2020 bid, and a dose of affirmation amid the oft-factious "Medicare for All" debate.

Barkan, who's suffering from Lou Gehrig's disease, declared his support for Warren in an op-ed and in a video posted to Twitter that took the form of a visual letter to his newborn daughter, Willow.

"This year, we are in a struggle for the future of our democracy," Barkan said, his voice dubbed over footage of intimate family moments, which are woven together with footage of Capitol Hill protests, President Donald Trump and Mitch McConnell. "That's why your mom and I are supporting Elizabeth Warren for President. She has the brains, brawn and moral clarity to overcome the challenges we face."

"On many days," he continued, "it feels like our democracy is too broken to address these enormous challenges, so your mom and I had to think hard about whether to bring you into this world. We knew you would not have a carefree childhood ... that I might not be around to watch you grow. And we knew your generation would have to fight to keep this world alive. But we decided to be hopeful and brave."

Barkan's endorsement comes less than a week after Warren rolled out her transition plan for Medicare for All, which arrived to mixed reviews -- some accused her of wavering and walking back her support for Medicare for All, while others expressed doubts on how long it could take to fully enact.

Moreover, Barkan's support comes mere hours before the fifth democratic debate in Atlanta, at which Warren will share the stage with Bernie Sanders, who developed Medicare for All and wrote the Senate bill. Barkan's also long been fond of Sanders.

Barkan has met with several 2020 candidates in his advocacy work, including Sanders and Warren. Barkan's op-ed praised each of their plans, and in what's seen as nominally an endorsement piece, he goes to great lengths to explain his choice of Warren over Sanders.

"I vividly remember wondering, on that tragic November night, whether I was going to die under President Donald Trump," he wrote. "For progressives like me -- and maybe you -- the choice in this primary is between Warren and Sanders. It is a difficult and wonderful choice to have."

He called himself an "admirer" of Sanders, and said he has "schemed" with him and campaigned by his side.

"I have no intention to diminish his incredible work building our progressive movement, or the ways in which his historic campaigns for president have shifted American political discourse," Barkan wrote. "He has done more than anyone else to build the movement for Medicare for All. He is a human being, with human shortcomings -- just like Warren. But I don't want to highlight those or criticize him, because I think he, like she, would be a transformative president."

Barkan also has a long history with Warren. In 2012, he launched a pressure campaign on the Federal Reserve in support of low-income people affected by monetary policy, calling Warren a "key partner" in that effort.

"I've worked with Elizabeth since before I was sick," Barkan wrote. "In 2020, we face a battle with fascism for the future of our democracy and our planet. Before that, we must fight a corporate establishment for the soul of the Democratic Party. I believe that Warren is the leader we deserve for those battles. You may prefer Sanders, and I have deep respect for that choice. ... We are, ultimately, on the same side."

Thanking Barkan, Warren tweeted, "I'm deeply grateful to have your support in this fight, @AdyBarkan. I make this promise to you, to Willow, and to every family in America: I will fight for your family as hard as I fight for my own. We're going to do this -- side by side. Together," Warren said, adding that "we wouldn't be this far in the fight" for Medicare For All without activists like him.

Barkan's endorsement of Warren over Sanders won sharp rebuke from Sanders supporters online Wednesday, but Sanders himself acknowledged Barkan's choice, expressed humble gratitude for Barkan's work and signaled to supports to stand down.

"There are very few people in the country doing more to make Medicare for All a reality than @adybarkan. I'm proud to call him a friend and I look forward to fighting alongside him to guarantee health care for all Americans," Sanders tweeted Wednesday morning.

Barkan has gained national fame fighting for equitable health care while battling through a painful, debilitating disease, but also for, in 2017, confronting then-Sen. Jeff Flake, R-Ariz., on a plane and urging him to vote against a crucial tax bill.

ABC News' Cheyenne Haslett contributed to this report.