Leah Wilson’s small organization was less than a year old when she realized she was making a difference. It was the beginning of 2020, just before the Covid shutdowns, and hundreds of protesters had gathered to demonstrate outside the State House in Trenton, N.J. They held signs with slogans like “my child, my choice” and “hands off our kids,” urging politicians to vote against a bill that would end religious exemptions for school-mandated vaccines.

Ms. Wilson, 38, wasn’t there. She was almost 700 miles away in her home state of Indiana. But more than 80,000 people had used her online platform to send messages directly to legislators. The measure ultimately fell short of passage by a single vote.

It was an outcome “no one thought was possible,” Ms. Wilson said, but her side had won.

Ms. Wilson’s organization, Stand for Health Freedom, has become part of a grassroots push in the years since. Hers is just one of many groups dedicated to the cause of “medical freedom,” a catchall term for ideas that often diametrically oppose scientific consensus and established medical practices. The movement has brought in people of various political persuasions, and Ms. Wilson considers her own organization “transpartisan,” though most of the candidates it endorses are Republican. To Ms. Wilson, those involved have coalesced around one idea: “There’s roles for government, and telling us how to care for our bodies is not one of them.”

The medical freedom movement represents people with a broad range of positions. Many want to reduce Food and Drug Administration oversight and see the United States exit the World Health Organization. They’re often resistant to proven public health measures like mask mandates and water fluoridation, and they support access to raw milk, despite the health risks associated with it. (On the issue of abortion, Ms. Wilson is anti-abortion but said she preferred that her group avoid the “very charged political issue.”)

Perhaps more than anything else, “medical freedom” has come to serve as a rallying cry for people who not only oppose vaccine mandates, but also see them as un-American. Ms. Wilson said she and her organization strove to be “protectors of freedom.” Her advocacy, she said, is ultimately about asking one question: “Do we truly believe that the American experiment of freedom is a worthy one? I do.”

Since the pandemic, the movement has gained momentum. Anti-establishment sentiment erupted as a result of Covid-related shutdowns and edicts. Medical freedom groups in places like Texas and Mississippi gained influence and secured victories, including around the issue of religious exemptions to vaccine mandates. The groundswell of interest energized Ms. Wilson’s small group as well. Though it has just two full-time employees and an annual budget around $400,000, the organization has used the internet to exercise outsize influence. It has issued 520 calls to action, according to the organization’s latest report, and had 72 “legislative wins” in which the vote went its way. More than 700,000 people have sent close to six million missives to legislators through the platform.