If we end up with a 50-50 US Senate- Democrats can gain another additional seat before 2020.

AK- Murkowski-R or Sullivan-R resigns. Begich-D appoints replacement.
CO- Gardner-R resigns. Polis-D appoints replacement.
FL- Rubio-R resigns. Gillum-D appoints replacement.
GA- Isakson-R or Perdue-R resigns. Abrams-D appoints replacement.
IA- Grassley-R or Ernst-R resigns. Hubbell-D appoints replacement.
KS- Roberts-R or Moran-R resigns. Kelly-D appoints replacement.
LA- Cassidy-R or Kennedy-R resigns. Bel Edwards-D appoints replacement.
ME- Collins-R resigns. Mills-D appoints replacement.
MT- Daines-R resigns. Bullock-D appoints replacement.
NC- Burr-R or Tillis-R resigns. Cooper-D appoints replacement.
OH- Portman-R resigns. Cordray-D appoints replacement.
OK- Inhofe-R or Lankford-R resigns. Edmondson-D appoints replacement.
PA- Toomey-R resigns. Wolf-D appoints replacement.
SD- Thune-R or Rounds-R resigns. Sutton-D appoints replacement.
WI- Johnson-R resigns. Evers-D appoints replacement.

Democratic US Senators from States with Republican Governor's.
AL- Jones-D
IN- Donnelly-D
MD- Cardin-D/Van Hollen-D
MA- Warren-D/Markey-D
MO- McCaskill-D
NH- Shaheen-D/Hassan-D
WV- Manchin-D