State legislators across the country are moving to tighten limits on vaccine exemptions for children amid one of the worst outbreaks of preventable diseases in recent history.

In response, an unusual coalition of libertarians and liberals influenced by a debunked study has launched a loud and at times threatening campaign to protect the statutory exemptions. A few instances required police intervention.

Legislators in 26 states have introduced bills to crack down on parents who request vaccine exemptions for either personal or religious reasons. Three states — Washington, Maine and New York — have passed those measures. A fourth — California — is likely to pass its version next month.

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But the debate has become personal, bitter and in some cases threatening, hijacked by a small but angry coalition that wants to allow unvaccinated children into schools and daycares.

When a California state Senate committee considered limiting exemptions based on personal or religious beliefs, vaccination opponents held signs showing blood on the face of the bill's chief sponsor. Some vaccine skeptics mailed bricks to legislators.

"What's that supposed to mean? They threaten, they bully because they don't have science on their side," said California state Sen. Richard Pan (D), the bill's primary sponsor and a practicing pediatrician. "They threaten me all the time. I'm fortunate. We do have security in the Capitol. The local police department, they're aware."

The rush of legislation comes against the backdrop of the worst outbreak of the measles virus in more than a generation. In the first six months of the year, the Centers for Disease Control and Prevention reported 1,095 cases of measles across 28 states.

The number of cases is nearly three times higher than at the same point last year, driven by significant outbreaks among religious groups in New York and suburban populations with less of a tie to organized religion in states such as Washington and Michigan.

"Only this year was the impetus front and center. When you have vaccine-preventable diseases like measles to the tune of 970 confirmed cases since September in New York, you have to do something about it," said New York state Sen. Brad Hoylman (D), who sponsored a bill limiting exemptions signed into law by Gov. Andrew Cuomo (D).

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"I felt it was important to push back on the anti-vaxxers who were fighting this battle tooth and nail," Hoylman said. "Had we not passed the legislation that we did, we would have conceded evidence-based ground to them, and would have essentially baked in the notion that vaccines are to be questioned. That's just not the case."

Those who oppose state requirements that their children receive vaccinations before they attend school or daycare, the so-called anti-vaxxers, are a small but vocal minority made up of libertarians who do not want government intervening in their lives and parents who might be influenced by celebrities such as actress Jenny McCarthy. They demonstrate, flood phone lines and fill committee rooms.

An anti-vaccination group called the Florida Freedom Alliance is planning what it calls a "millions march" on Tallahassee in August. The California Coalition for Vaccine Choice, which opposed Pan's bill, is organizing demonstrations in Sacramento for next month.

Attempts to reach the California Coalition for Vaccine Choice, which paid for the posters showing Pan covered in blood, went unreturned last week.

"There's always been anti-vaxxers, but generally they've been on the fringe," Pan said in an interview. "Anyone who wants their own children to get these diseases obviously has no knowledge of how serious these diseases are."

Some publicly elected vaccine opponents are now seeking higher office.

Kentucky state Sen. Ralph Alvarado (R), a practicing physician running for lieutenant governor alongside Gov. Matt Bevin (R), said last month he would not require people to receive vaccines even though he recommended they get their shots. Bevin, a Tea Party Republican, said he exposed all nine of his children to chickenpox, a practice medical experts have called dangerous.

Those trying to tighten vaccine exemptions say they take their cues from Pan, who first won a seat in the California legislature in 2010. Since Pan’s first measure limiting vaccine exemptions passed in 2015, a dozen other states have followed California's lead.

"I took a page out of the book of California," Hoylman said. "We can be preventing these outbreaks just as California did."

Vaccines work best when a sufficient percentage of the population is immunized against a disease so that it does not have the ability to infect new individuals and replicate. That threshold, known as herd immunity, is when a virus begins to starve and die out.

But not everyone can be vaccinated. Some people with compromised immune systems who might have cancer or an autoimmune deficiency cannot receive vaccines for medical reasons. They are then at risk if they encounter someone with a personal or religious exemption who has contracted a disease such as measles.

"This is not a matter only of the rights of those who profess to have religious beliefs, but it's also the right of parents and children to be educated in a school where they feel safe," Hoylman said. "Why should immunocompromised young people be afraid to go to school?"

Vaccines can be effective to the point that people begin forgetting the horror that a preventable disease can cause. Most of the 13 colonies required early Americans to receive a smallpox vaccine after it was introduced in 1796. In 1905, the Supreme Court ruled in Jacobson v. Massachusetts that a state had the right to enforce mandatory vaccination laws.

Over the intervening century, most states passed laws allowing some form of nonmedical exemption for vaccines. Today, 45 states and the District of Columbia allow at least some recourse for someone with an objection to their child receiving vaccines.

The number of parents who seek and obtain those exemptions has been rising for the past two decades, fueled in part by a debunked study published in 1998 by a British doctor named Andrew Wakefield, who fraudulently linked vaccines to autism and who has since had his medical license revoked.

That study, medical historians say, prompted a panic among parents who declined to vaccinate their children in both Europe and the United States, creating a population that some call the Wakefield generation. Another subgeneration that is slightly older is now made up of those who are resisting vaccines for their own children.

"You had people who were seeing their children being diagnosed with autism and not really having a reason why," said Rene Najera, a public health doctor and editor of the History of Vaccines website, a project of the College of Physicians of Philadelphia. "Along comes Wakefield with his fraudulent study and says it's the vaccine. It seemed plausible to them."

Social media has made the problem worse, as demonstrably false information makes its way around the internet faster than the debunking documentation.

But cracking down on exemptions has worked in the past. A study published this month by the Journal of the American Medical Association found that the number of unvaccinated kindergarteners in California dropped by half after lawmakers passed Pan's 2015 law limiting some exemptions after a measles outbreak at Disneyland.

Najera said the few states that have held firm against religious or personal belief exemptions remain models for the country. West Virginia and Mississippi — states with the highest levels of vaccinated populations in the country — have weathered the recent storm much better than others.

The measles outbreak that has raced across the United States in the past six months has not infected anyone in either West Virginia or Mississippi.