Many cases reported in the United States this year were linked to international travel, according to the C.D.C., as travel destinations such as Britain, Austria and the Philippines have had outbreaks. Many of the people in the United States who have been infected have been unvaccinated children age 12 months and older.
I think you already know
This seems like a non-partisan thing that only became partisan recently. Look at RFK Jr, a lifelong Democrat who pushes anti-vaccine misinformation.
Here’s how I see events:
If Democrats were in power, they likely wouldn’t have pushed for mandatory vaccination, so the anti-vaccine crowd probably wouldn’t have allied with the GOP (I’m guessing the GOP wouldn’t criticized slow development of the vaccine or something).
Most people are vaccinated, so vaccination is absolutely the majority in both parties. Anti-vaccine groups are like any other conspiracy group, they’ll join with whichever larger group seems to be more sympathetic at the time.