People always assume that just Republicans are always bound to do well in midterms just because. If anything, midterms are a referendum on the incumbent president / majority party, and if things really go down south for Trump and the Republicans (and trust me, the last two weeks haven't been good for them whatsoever), then expect a Democratic wave year.
At very very minimum I think Democrats will be able to compete in a lot my territory outside their bases in 2018. A year in which Democrats gain like 14 house seats, several governorships, and make headway in state legislatures but lose 4 to 5 seats in the senate will still be defined by the media as a gop leaning year. Which would be insane because the Senate elections themselves are staggered and quite insulated.
Actually what I just wrote seems to be a very likely outcome at this point.