Ady Barkan, an influential liberal activist who has ALS, endorsed Sen. Elizabeth Warren for president Wednesday, giving her a boost in the sometimes contentious Democratic debate over health care and Medicare-for-all.

“She has the brains and the brawn and the moral clarity to overcome the challenges that we face,” Barkan said in a video. “I’ve seen up close how she confronts a problem. She listens to the people most affected, she does her homework and then she comes up with a plan. A brilliant, workable plan.”

The endorsement comes hours before a Democratic debate and as Warren tries to navigate the rough political currents surrounding Medicare-for-all. Facing increased pressure in recent weeks, the Massachusetts senator released a financing plan for her health-care proposal that included some tax increases on large corporations and the very wealthy, then a “transition” plan that critics said amounted to backing away from Medicare-for-all.

Barkan, who has been in contact with the Warren campaign, praised each of those plans despite the controversy around them, facing harsh criticism online from supporters of Sen. Bernie Sanders (I-Vt.), with whom Barkan is still close.

In an op-ed for the Nation that accompanied his endorsement of Warren, Barkan said he remained an admirer of Sanders and that deciding between the two was “a difficult and wonderful choice to have.”

On Wednesday morning, Sanders signaled to his supporters they should back down from attacking Barkan.

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

Barkan was already active in liberal causes when he was diagnosed three years ago with amyotrophic lateral sclerosis, also called ALS or Lou Gehrig’s disease, a neurological illness for which there is no cure. ALS patients gradually lose the ability to move, speak, swallow and breathe.

Barkan gained national attention in 2017 after confronting then-Sen. Jeff Flake (R-Ariz.) on an airplane to urge him to vote no on a major Republican tax bill. “What should I tell my son, or what would you tell my son, if you pass this bill and he cuts funding for disability and I can’t get a ventilator?” he asked Flake.

That willingness to speak out while fighting the debilitating disease has won him many followers and considerable clout in the Democratic Party. Over the past year, several of the presidential candidates have met with Barkan to discuss health care.