If I were one of those Republicans in California I'd be pissing myself now. It's always bad to be in the party of a newly elected President during midterms but Trump + California might be the perfect storm for a Republican bloodbath there.
Problem is Hispanic turnout tanking in midterms. The Central Valley seats, in particular, seem to jump 6-7 points to the right every mid-term.
Also students and young people in general. For example, Mimi Walters district in OC contains UC Irvine, a very large university. Turnout will likely be down there in 2018 and the Democrat needs a nice margin there if they have any hope of defeating Walters. Also, Orange County is exactly the type of place that votes for Democrat for President or Governor vs down ballot. Besides Darrell Issa, I am not that optimistic about any of the other OC Republicans being defeated anytime soon. Perhaps in 2020 if Trump is defeated with a healthy margin.