The Dems have a tough Senate map in 2018.
They have to defend about 25 Senate seats in 2018, but the Republicans have only about 10 up for re-election.
A number of Democrats up for re-election are in states won by Trump.
And, for unknown reasons, historically, voter turnout is lower in the mid term elections among Democrat inclined voters.
Add it up, and the Democrats seem likely to lose a few Senate seats. They may, just maybe, pick up a few House seats, but unlikely enough to tip the balance there.