David Wasserman in FiveThirtyEight:
When Democrats think about their party’s problems on the political map, they tend to think of President Trump’s ability to win the White House despite losing the popular vote and Republicans’ potent efforts to gerrymander congressional districts. But their problems extend beyond the Electoral College and the House: The Senate hasn’t had such a strong pro-GOP bias since the ratification of direct Senate elections in 1913.
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.
But there’s a larger, long-term trend at work too — one that should alarm Democrats preoccupied with the future of Congress and the Supreme Court.