They will do what they always do and make local elections national. It won't be about their district or their state...it will be a referendum on Trump in the same way they made 2006 a national referendum on Bush 43.
And unless something like Obamacare repeal (full repeal not a half measure) and/or some meaningful tax cuts are passed before the mid terms...it may end up about as well for Trump as it did for W.