Is Justice vulnerable to a GOP Primary?
It's pretty rare when a GOVERNOR of a state changes parties. That's far more rare than a legislator changing parties, and that's because the GOVERNOR is the leader of his party in his/her state for the most part.
How many Governors switched parties while in office? The only one I can remember is Buddy Roemer who switched to the GOP in 1991. That didn't work out well for Roemer, whom I believe would have squeaked out reelection as a Democrat (or may have been resoundingly reelected if he was the candidate squaring off against David Duke in 1991). Justice was a lifelong Republican who became a Democrat in order to run for Governor and switched back after less than a year in office, but I can't believe that this sat well with GOP regulars in WV. Justice is vulnerable in a primary on general principles, given the manner in which he switched to the Democrats to get elected, and then switched right back to the GOP. Party regulars don't like being sidestepped like that.
Roemer was elected LA Governor in 1987 defeating EWE, who got his revenge in 1991.
EWE won 4th term in a massive landslide.