2020 is going to be the year Democrats regain control of the US Senate.

2018-
Best Case Scenario for Democrats is they pick up AZ and NV but lose ND. +1D 50D 50R
Worse Case Scenario for Democrats is they don't pick up any seats and lose ND, MO and IN. -3D 46D 54R
2020
Democrats need a net gain of 5 seats to regain control of the US Senate.
1 Democratic held US Senate seat up for re-election in 2020 is more likely than not to go Republican
AL- Jones-D assuming Republicans don't nominate another controversial candidate.
Democrats need to win 6 Republican held seats up in 2020 to regain control of the US Senate.
The Republican held US Senate seats up in 2020 that are highly vulnerable are:
AZ special
CO
GA
IA
LA if Mitch Landrieu-D runs.
ME if Collins-R retires or loses in the primary.
MT if Bullock-D runs.
NC