In the last 20 years, Senate candidates have tended to win or lose in tandem with the presidential candidates of their party. For the Republicans trying to hold their control of the Senate in 2020, that may be a mixed blessing. President Trump — whether he wins or loses his own re-election bid — will have a profound effect on crucial Senate races in Arizona, Colorado, North Carolina and Montana.
If Mr. Biden wins, the Democrats will only need to pick-up three Senate seats to seize control. To maintain the majority, Republicans will need good Senate candidates to run good campaigns in a handful of states.