I'm not sure where this notion comes from. Either way, West Virginia law requires appointees to fill vacancies in statewide office to be of the same party as the last officeholder, selected from a list presented by the state party, so there's no threat that his seat would be gone prematurely for the Dems if he were to resign. Having no idea who the party would nominate, there's no way of knowing whether his replacement would be any different from him, but it's an empty threat either way.
There is if he resigned before the midterms: the GOP would just pick the seat up 2 years early in this scenario's special.
