If there's one thing in America you can actually be unequivocal on, it's vaccinating children against diseases like measles. It is an uncomplicated issue.

And yet, a handful of top Republicans have opened the door slightly on the subject: Parents "need to have some measure of choice in things," Chris Christie said in London after saying his own kids were vaccinated, "so that's the balance that the government has to decide."

"What happens if you have somebody not wanting to take the smallpox vaccine and it ruins it for everybody else?" Rand Paul asked on Laura Ingraham's radio show, before continuing, "I think there are times in which there can be some rules, but for the first part it ought to be voluntary."

"I have heard of many tragic cases of walking, talking normal children who wound up with profound mental disorders after vaccines," he told CNBC.

"I think parents have to make choices for their family and their children," Carly Fiorina told BuzzFeed News.

Observers have rolled their eyes with disgust and familiarity: These politicians are seeking the Republican nomination for president, and the primary process does often involve pandering to fringe elements of one's own party.

But who are these people pandering to? What part, exactly, of the Republican coalition so opposes mandatory requirements that, in the context of a measles outbreak, vaccination is a compromise issue?

Because here's the truth: This is largely a liberal fringe issue.

The people not vaccinating their kids against the diseases once declared defeated don't live in South Carolina or Indiana or a particularly conservative part of Ohio or Florida. That isn't where people are contracting the whooping cough, like it's goddamn Little Women.

No, the strongholds are in places like Newport Beach, Santa Monica, and Marin County, California. The affluent, the educated, the enlightened, the ones who believe in purity and science — people in liberal enclaves are the ones rejecting one of the 20th century's major scientific achievements.

Half of the children that attend some schools in Marin (median income: $90,839), the county's health officer told the New York Times last week, are unvaccinated. People don't want toxins in their children's blood.

"It's good to explore alternatives rather than go with the panic of everyone around you," the mother of two unvaccinated children told the Times. "Vaccines don't feel right for me and my family."

Now, as of 2011, 19 states had "philosophical" exemptions — the way you might oppose designated hitter, except in this instance, you oppose a medical professional immunizing your child from rubella, and cool, that works.

Many of those states, with a fair amount of exceptions like Texas and Louisiana, conjure the image of people hiking in fleeces. California, Vermont, Washington state, Maine, New Mexico, Colorado. These are states where one can oppose vaccination for personal reasons.

And Democrats do in fact pander to these people. In fact, in 2008, both Hillary Clinton and Barack Obama gestured in the direction of the discredited link between autism and vaccines (though whether Obama actually pandered is in dispute).

Now, a YouGov poll last week (on smallish party ID samples) put Republican support for mandatory vaccination requirements at 57%, significantly lower than Democratic (70%), and higher than independent (48%). And maybe there is some inherent, liberty-based resistance to the word mandatory, or that's a lingering, socially conservative byproduct of the protracted 2011 debate over a Texas law that had mandated the HPV-vaccine for young girls.

But it's easy to confuse answers to back-of-mind poll questions with real, organized, political constituencies — and when opposition to vaccination is happening in an organized, robust way, it's happening in parts of the country with very little conservative appeal.

And so this is actually the perfect, almost platonic ideal political setting for a Republican to say very simply and end it there:

Vaccinate your kids.