Zuckerman shot back that Holcombe is trying to use a public health crisis to score political points.

“The fact that any political campaign is trying to use this moment for political opportunism is unconscionable,” he told POLITICO. “Right now, my primary focus is to disseminate important health information about the virus and how to keep Vermonters, their families and our community safe...When the COVID-19 vaccine is available for the coronavirus it should be free for all and universally accessible.”

The fight in Vermont’s Democratic gubernatorial primary sets up a larger test of how mainstream the “anti-vaxxer” movement has become on the left, and whether the coronavirus pandemic could make it politically untenable even in liberal bastions.

Anti-vaccine advocacy is a growing force in American politics and around the globe as formerly fringe activists have developed a wide reach across social media platforms. Other candidates who oppose mandatory vaccinations are running for office, and some are winning, with support from large and well-funded advocacy groups.

Recently-ousted Kentucky Gov. Matt Bevin has said that mandatory vaccines are un-American. Sen. Rand Paul (R-Ky.) has repeatedly said parents should be able to decide whether or not to vaccinate their children. With backing by the group Texans for Vaccine Choice as well as Republican Gov. Greg Abbott, libertarian Susanna Dokupil is making another run for the state legislature, primarying one of the most vocally pro-vaccine Republicans in Austin.

But the movement has had even more success in blocking pro-vaccine legislation, defeating efforts in New Jersey, California and other states that would have eliminated exemptions to immunization.

Zuckerman, 48, has faced fierce criticism from Republican opponents in past races for his position on vaccinations, yet still won. He is seen as the frontrunner in an August primary for the Democratic nomination, though Scott is favored to win reelection. But Holcome is betting that the anxiety surrounding coronavirus will make the issue more salient and perhaps will force a reckoning with the anti-vaxx left. That movement includes several high-profile celebrities along with former presidential candidate Marianne Williamson, a vocal skeptic of mandatory vaccinations, campaigned for Sanders after she dropped out of the race.

Last year, the World Health Organization labeled what it called “vaccine hesitancy” one of the top 10 threats to global health, citing a 30% increase in measles cases worldwide. Vaccine skepticism is often tied to populist political movements on the right and the left. It’s part of a bigger surge of anti-establishment anger around the world — including in the U.S., where less than half the population gets vaccinated against the seasonal flu, which has killed tens of thousands in the past year alone.

Despite past blowback for his positions, Zuckerman has remained firm in his opposition to mandatory vaccinations. He was called a “hero” by the co-founder of a Vermont “vaccine choice” group during his 2016 bid for lieutenant governor.

In 2015, while serving in the state Senate, Zuckerman strongly opposed a bill that repealed the “philosophical exemption” to vaccinations which ultimately passed. And in a 2018 debate while running for reelection, he defended his past positions by arguing the Center for Disease Control and Prevention has financial connections with the pharmaceutical industry that made him skeptical of its recommendations.

“The CDC’s infectious disease control board actually has a number of conflicts of interests,” he argued. “And so yes, like many, I do sometimes question when government agencies are a bit too infused with corporate influence with respect to some of the outcomes and decisions they make.”

Yet Zuckerman has thrived politically, and some Vermont Democratic officials speculate he could someday succeed Sanders, a longtime ally, in the Senate.

Sanders’ presidential campaign declined to comment on Zuckerman, but one aide noted that Sanders supports mandatory vaccinations with "very limited exceptions.” The aide said “there are some health exceptions“ but declined to provide further details on which exceptions he supports.

Many in the so-called “anti-vaxxer” community have tried in recent years to make their position more politically palatable by arguing that while they believe vaccines are effective, they are opposed to the government requiring them. Zuckerman has taken a similar messaging approach. In the 2018 debate, Zuckerman said “the science behind vaccines is sound, I think vaccines do good for our communities, my daughter is vaccinated. But it’s a question of whether government should be forcing that onto individuals.”

Public health experts say such arguments put vulnerable people at risk. Even before the recent coronavirus pandemic, several states had been moving to eliminate religious and other exemptions for childhood vaccinations, as measles and other diseases have resurged in recent years.

“That's a dangerous message, that everyone should decide for themselves,” said Lois Privor-Dumm, a senior researcher on global vaccine policy at Johns Hopkins University. “If [people who forego vaccines] want to keep themselves quarantined all the time, that’s one thing. But that’s not what happens in life. So it's not appropriate for candidates to go against all the public health experts out there.”

While campaigning in 2016 the Republican primary, Donald Trump also argued without evidence there is a correlation between vaccines and autism, a position which drew rebuke from the medical and scientific community as well as fellow candidate Ben Carson, a brain surgeon who now serves as his secretary of Housing and Urban Development.

As president, however, Trump shifted his position. In response to a number of Measles outbreaks in 2019, Trump told parents that their children “have to get the shots. The vaccinations are so important.”