Never mind Trump lost less than half the number of House seats his first midterm as Obama lost his first midterm. Gotta go looking for causes outside of normal trends.
I think what we need to be concerned with is the total difference in the number of votes in 18 was larger than in ‘10. That vote is now mostly concentrated in the once republican/safe burbs. If it grows into areas that are more country than suburban, then there may be a problem.