Ady Barkan, a well-known activist who campaigned for Medicare for all while struggling with the neurodegenerative disease A.L.S., died on Wednesday in Santa Barbara, Calif. He was 39.

His death, at Santa Barbara Cottage Hospital, was announced by Be a Hero, the political organization he co-founded in 2018.

Mr. Barkan was diagnosed with A.L.S., or amyotrophic lateral sclerosis, in 2016, four months after the birth of his son, Carl. The disease, which causes paralysis, strikes many patients in the prime of life and often leads to death within two to five years.

As Mr. Barkan confronted his mortality, he dedicated the rest of his life to changing the American health care system.

His profile and influence grew even as his health deteriorated, in part because he had a knack for blending his personal story with calls to action. He testified before Congress, interviewed Democratic presidential candidates and spoke at the Democratic National Convention.

“That’s the paradox of my situation,” he told The New York Times in 2019. “As my voice has gotten weaker, more people have heard my message. As I lost the ability to walk, more people have followed in my footsteps.”

Ohad Barkan was born on Dec. 18, 1983, in Boston. His mother, Diana Kormos Buchwald, is a professor of the history of science at the California Institute of Technology in Pasadena. His father, Elazar Barkan, is a professor of international and public affairs at Columbia University. Both emigrated to the United States from Israel.

Mr. Barkan was raised in Cambridge, Mass., where his parents were graduate students, and later in California, where he attended high school in Claremont. One of his first forays into politics was volunteering on an election campaign for Representative Adam Schiff, Democrat of California.

He met his wife, Rachael King, who is now a professor of English literature at the University of California, Santa Barbara, at Columbia University’s student newspaper when they were undergraduates there.

Initially wanting to be a lawyer, Mr. Barkan was a clerk for a federal judge in New York after law school. But he decided to become a full-time activist after being drawn to the Occupy Wall Street protests that began in Lower Manhattan in 2011.

Before his A.L.S. diagnosis, Mr. Barkan was an energetic but relatively anonymous foot soldier for progressive causes, including rights for immigrants and workers, ending mass incarceration and reforming the Federal Reserve. After getting sick, he became a hero of the left and a social media star. Politico called him “the most powerful activist in America.”

He was adept at attracting public attention to his progressive causes. On an airplane in 2017, he confronted Senator Jeff Flake, Republican of Arizona, over a Republican tax bill that he believed could lead to steep cuts in social services, like health care.

“Think about the legacy that you will have for my son and your grandchildren if you take your principles and turn them into votes,” Mr. Barkan said. “You can save my life.”

In 2018, he was arrested in his wheelchair in a Senate office building as he protested the Supreme Court nomination of Brett M. Kavanaugh.

Be a Hero, which was formally founded that year, eventually grew to include two nonprofits and a political action committee. Among other issues, the group campaigned to protect nurses during the pandemic, and to replace Senate Republicans, who it said were the chamber’s “most dangerous voices” in the 2022 midterm elections.

Senator Elizabeth Warren, Democrat of Massachusetts, said in September that she had watched Mr. Barkan “pick a lot of really good fights” over the years, and that he had been instrumental in stopping Republican efforts to repeal the Affordable Care Act, President Barack Obama’s signature domestic achievement.

“Thanks to his persistence, he hasn’t just been in the fight,” Ms. Warren said, speaking virtually to an audience at the Franklin D. Roosevelt Presidential Library and Museum in Hyde Park, N.Y., where Mr. Barkan was accepting an award from the Roosevelt Institute for his activism. “He’s been leading these fights, and helping win them.”

In the lead-up to the 2020 presidential election, Mr. Barkan made clear that while he endorsed the Democratic nominee, Joseph R. Biden Jr., he disagreed with the candidate on health care policy. (President Biden opposes Medicare for all, and Mr. Barkan had initially endorsed Ms. Warren and later Senator Bernie Sanders of Vermont for the party’s nomination.)

In a 2020 discussion with Mr. Barkan over Zoom, Mr. Biden would not commit to doubling the budget for the National Institutes of Health, saying that he would “significantly increase the budget” and ensure that “we spend another $50 billion on biomedical research” over the next several years.

“I think that is not enough,” said Mr. Barkan, who by that point could speak only through a computerized voice using eye gaze technology.

“Well, maybe when I get elected, you can come and help me figure out what’s enough,” Mr. Biden told him.

“Thank you, Mr. Vice President,” Mr. Barkan replied. “I’ll take you up on that.”

Mr. Barkan is survived by his parents; his wife; their children, Carl, 7, and Willow, 3; a brother, Muki Barkan; and several aunts and uncles.

In a video celebrating Mr. Barkan’s 39th birthday, Carl summarized his father’s life’s work with remarkable economy: “He helps to make sure it’s not too expensive for people to go to the doctor.”

Mr. Barkan remained relentlessly optimistic and energetic even as he become paralyzed from the head down and lost control of his breathing. In 2018, he traveled to 22 states in 40 days. Three years later, he argued in an opinion article in The New York Times that home- and community-based care deserved more federal funding.

“Although I’m not the father I had hoped to be, I’m grateful for each moment with my children,” he wrote. “And it’s all possible because I have 24-hour home care.”

In a speech at the Roosevelt Library in September, his last in-person event, Mr. Barkan opened by thanking his three caregivers and saying that he and Ms. King would soon celebrate the 18th anniversary of their relationship.

“Every year has been an adventure, and coming to New York this week, especially with our two perfect angels, Carl and Willow, is wonderful evidence that new adventures still await us,” he told the audience from his wheelchair. “And that staying in the struggle can bring beautiful rewards.”

Johnny Diaz contributed reporting.