Mid-term elections are always about the incumbent president — at the margin. That is, some voters (probably “swing” voters in the general elections), cast their House and Senate ballots in protest against the presidential candidate for whom they had voted in the preceding general election. (It’s like having a tantrum when the tooth fairy doesn’t leave as much as one had expected.)
The post-World War II record for the House, where all seats are on the line in every election, tells the story. The GOP usually fares worse in mid-terms when there’s a Republican in the White House than it does when there’s a Democrat in residence.
In the first graph below, you can see that Republicans have won more than 50 percent of House seats in only 6 post-WWII mid-terms, and 5 of those wins occurred during a Democrat presidency. The mirror image view is similarly lop-sided: Democrats have won more the 50 percent of House seats in 12 post-WWII elections, and 8 of those wins occurred during a Republican presidency.
In the second graph below, you can see that Republicans gained House seats in 9 mid-term elections, and 8 of those elections were held when a Democrat presided. The mirror image: Democrats gained House seats in 9 mid-terms, 8 of which occurred during a Republican presidency.
So if the GOP loses the House in 2018, it will be in line with history and not necessarily because of Trump — though the pundits will play it that way.