What is going on? The vast majority of Americans of either party support vaccinations. However, people in the two parties are moving in opposite directions with more Democrats favoring vaccinations and fewer Republicans doing so--still at 65% or so.
Perhaps the best pithy diagnosis of this is by Howard Dean, who is a doctor as well as a prominent Democrat:
The last line explains why the anti-vaxxers tend to be people on the far left and far right. If these focus are fringe, why are the GOP candidates pandering to them? Because the American primary process, especially in the Republican Party, means playing to the base and particularly to those in the base who are most passionate and turn out. Pro-vaxxers are not going to vote on that issue. But anti-vaxxers? Absolutely. This leads to outbidding--that the GOP candidates will compete with each other to be the most suspicious of government. Lovely.Howard Dean, a presidential candidate in 2004 and a former chairman of the Democratic National Committee, said there are three groups of people who object to required vaccines: “One is people who are very much scared about their kids getting autism, which is an idea that has been completely discredited. Two, is entitled people who don’t want to put any poison in their kids and view this as poison, which is ignorance more than anything else. And three, people who are antigovernment in any way.”
On the other side? Perhaps Hillary Clinton is not so worried about being outflanked:
The science is clear: The earth is round, the sky is blue, and #vaccineswork. Let's protect all our kids. #GrandmothersKnowBestI have seen some stuff lately indicating that the most conservative candidates cannot win the primary (Huckabee?), but that ignores the reality that all GOP candidates get pushed further to the right by the outbidding dynamics. So, the most Conservative may not win, but anyone who does will have to establish a record of being very Conservative, which, ironically, means pursuing policies that are actually not conservative. Conservative usually means sticking with the tried and true, the old policies that seemed to work in the past. What fits that category better than vaccines?
— Hillary Clinton (@HillaryClinton) February 3, 2015