From the liberal pollster Nate Silver.
Even if Democrats were to win every single 2018 House and Senate race for seats representing places that Hillary Clinton won or that Trump won by less than 3 percentage points — a pretty good midterm by historical standards — they could still fall short of the House majority and lose five Senate seats.
This is partly attributable to the nature of House districts: GOP gerrymandering and Democratic voters’ clustering in urban districts has moved the median House seat well to the right of the nation. Part of it is bad timing. Democrats have been cursed by a terrible Senate map in 2018: They must defend 25 of their 48 seats1 while Republicans must defend just eight of their 52.