It’s my understanding that Morrissey (I think the current Secretary of State!) is leading in the GOP primary. I hear he had a good shot at taking out Manchin. Probably the source of Manchin’s angst!
If he wanted to be a Republican, he could have switched in 2017. Now he’s facing reelection and knows his seat is at risk. A Democrat who could be trusted to switch would have helped the administration on the bills that have come to the floor, and Senator Manchin has not.