A GOP senate, court and president is a nightmare while himself in there. The GOP have shown they are happy for him to be the mad king so long as they can have oil and guns and shitty attitudes. As far as i'm concerned he sees himself as president for life and will use this as the stamp of approval to pursue that, and the GOP will support it.
It is very unlikely that the democrats could take the senate, best (and unlikely) they could do is 50 50 but pence would then be the tie breaker so republicans would still control. 25 of the 35 senate seats up for election are democrats held seats.
They could take control of the house of representatives though I remember seeing an estimate they need to win the popular vote by 7% to take control of the house
The governor races are important in terms of redistricting in 2021 or so