Bucky wrote:i guess JH is best suited to answer this: when does trump support become a liability in an R primary?
Probably never in a Republican primary. Not any time soon anyway. Did a poll last week in a state Trump won pretty comfortably but among likely 2018 GOP primary voters his approval was at like 82% with 60%+ strongly approving. I doubt there's a state where he's under 70% approval among GOP primary voters. Like the week Nixon resigned after a year+ of negative headlines he still had 50%+ approval among Republicans. That said most places the local elected Republican is also strongly supported by the primary electorate and they have goodwill separate from Trump that could withstand attacks from Trumpish challengers.
The angle that matters I think are districts where the R member has to be more concerned about the general than the primary, and that number grows as Trump becomes less popular, which increases incentives for breaking on specific issues or working to hold the admin to account on ethics things.