The WV Mountain Party will presumably attack Manchin a lot for voting for Pruitt.
Probably, he could be primaried. But for now, a Trumpist approach to enviromentalism is winning in West Virginia. I'm betting on Manchin doing a similar thought. Personally, at least for now, I think an enviromentalist candidate beating Manchin in WV Dem Senate primary would end up in GE like Akin or Mourdock on the opposite side.
He nominated for governor a Capt. Planet villain who was a Republican until 10 minutes before the election and won by a large majority, sadly his position on coal will be very popular in WV for a few more years at least