Midterm Losses: the president's party usually loses seats in the House, often in the Senate as well.
Approval: the losses are more severe when the president is unpopular. And Trump is unpopular.
The House the generic ballot, and seat ratings
Lots of R retirements
Lots of D candidates, with money
This Tuesday, watch a special election in Arizona.
Senate races: the map constrains Democratic gains
Governorships