Though the stereotype of a vaccine skeptic is a coastal, back-to-the-land type, it's generally been Democratic-controlled states that have tightened vaccination laws. This week, Democrats in two of those states — California, where a measles outbreak was traced to Disneyland, and Washington state — proposed eliminating laws that allow parents to opt out of vaccination for personal reasons.
Meanwhile, in Maine, Republicans are objecting to a similar effort. In Minnesota, only Democrats have signed on to sponsor a bill to make it harder to avoid vaccinating children. And last year in Colorado, it was largely Republicans who squashed an effort to force parents to get a doctor's approval if they chose not to inoculate their kids.
"This boils down to, does the government force everyone to conform or do we empower everyone to make decisions on their own?" said Colorado state Sen. Kevin Lundberg, a Republican who did not fully vaccinate his children and led the fight against last year's bill.
This year, Colorado Republicans introduced a bill stating that parents have the right to make all medical decisions for their children, legislation that was cheered by vaccination opponents Thursday at a hearing.
American skepticism toward vaccinations dates back at least to the Revolutionary War, when George Washington was initially reluctant to inoculate his troops against smallpox.
"There is a long history to the fight against vaccination, and it does seem to break down along liberal versus conservative lines," said Kent Schwirian, a sociology professor at Ohio State University. He surveyed people during a 2009 swine flu scare, asking whether they would get vaccinated, and found that conservatives who distrusted government were less likely to support inoculation.
Modern-day vaccine skeptics are hard to pigeonhole politically.
"We're the bridge between the granola moms and the stiletto moms," said Dotty Hagmier, a nurse whose group Moms In Charge includes vaccine skeptics.
A Pew poll found that Republicans are slightly more likely than Democrats to oppose vaccine mandates. In legislatures, the issue has not been purely partisan, with some Democrats joining Republicans to fight vaccine requirements and some members of the GOP pushing to increase vaccinations.
The administration of Michigan Republican Gov. Rick Snyder last year required parents to go to a local health department before they could skip vaccinations for their children. In Vermont, Democrats and Republicans alike scaled down a bill in 2012 that would have limited parents' options. Still, people on both sides see a divide forming.
Diane Peterson of the Immunization Action Coalition, which tracks vaccination legislation nationwide, said she has had difficulty getting Republicans to sign on to a bill to expand vaccinations in her home state of Minnesota.
"I don't feel this should be a partisan issue in any way, and it's disturbing that it is," Peterson said.
Vaccination politics erupted into the national spotlight this week when three Republican presidential contenders — New Jersey Gov. Chris Christie, Kentucky Sen. Rand Paul and former Hewlett-Packer CEO Carly Fiorina — seemed to side with parents who oppose vaccinating their children.
Several other prominent Republicans, including House Speaker John Boehner, spoke out in support of vaccinations, and conservatives noted that Hillary Clinton told an anti-vaccine group in 2008 that she wanted more research into the discredited theory that inoculations cause autism.
Barbara Loe Fisher of the National Vaccination Information Center said she sees no partisan divide.
"This has always been a bipartisan issue," said Fisher, whose organization fights vaccine mandates. "The demographic spans the political spectrum."
The main difference, she said, is that her side has far less money than the pharmaceutical companies and medical groups that she blames for the push to tighten vaccination requirements.
In Maine, Democratic state Rep. Ralph Tucker has proposed legislation to end that state's exemption for parents who are personally opposed to vaccinating their children. But Republican Rep. Deborah Sanderson said her party dislikes the idea.
"If you're going to mandate that a parent do something that philosophically they are opposed to, that's a tremendous overreach of government," Sanderson said, adding that she still thinks vaccinations are important.
Vaccination opponents are teaming with Republicans — and, they say, some Democrats — to instead create a state office of vaccine safety to monitor problems with inoculations.
In Colorado, Democratic state Sen. Irene Aguilar, a doctor who sponsored the bill making it harder to opt out of vaccinations, said it was mostly Republicans who scuttled her proposal, but said that a few Democrats were wary too because they had grandchildren who were not vaccinated.
"This is another of those far left-far right issues," she said.
Neil Miller puts himself on the left side of the political spectrum. A resident of famously liberal Santa Fe, New Mexico, Miller favours organic food, made sure his children were home-birthed and supports left-wing Massachusetts Sen. Elizabeth Warren.
But he's also spoken widely about what he sees as the risks of vaccination, and he was happy to publicize the statements by Christie and Paul — two politicians he does not normally support — on Facebook.
"I'm very happy there's a segment of the conservative party that supports the rights of parents to make these decisions," Miller said.
Associated Press writers Brian Bakst in St. Paul, Minnesota; Lisa Bauman in Helena, Montana; Alana Durkin in Augusta, Maine; David Gram in Montpelier, Vermont; Rachel LaCorte in Olympia, Washington; and Juliet A. Williams in Sacramento, California, contributed to this report.