2020 will be the year Democrats regain control of the US Senate.
2018
10 Democratic incumbent US Senators up for re-election in 2018 are from States Trump carried in 2016
FL-Nelson-D
IN-Donnelly-D
MI-Stabenow-D
MO-McCaskill-D
MT-Tester-D
ND-Heitkamp-D
OH-Brown-D
PA-Casey-D
WV-Manchin-D
WI-Baldwin-D
Out of those 10, 5 of those States Trump carried by a double digit margin
IN,MO,MT,ND,and WV.
Democrats could hold onto MT but lose IN,MO,ND,and WV and pick up AZ and NV- Republican held seats in states Trump narrowly won or lost in 2018. -2D is a worse case scenario for Democrats in 2018. 47D 53R
2020
1 Democratic incumbent US Senator up for re-election in 2020 are from State Trump will carry in 2020
AL-Jones-D
Democrats have a 50-50 chance of holding onto AL
Democrats will pick up Republican held seats in AZ(special election to replace McCain-R),CO,GA and NC- States Trump will lose or narrowly win in 2020.
Democrats could pick up ME if Collins-R retires or loses in the primary and MT if Bullock-D runs.
Democrats will have a net gain of 3 to 6 seats in 2020.