The only class of Senators that has any real GOP vulnerability is the 2022 class.
2018: NV; Maybe AZ; Possibly TX, MS
2020: NC; Maybe AK, CO, IA; Possibly GA, MT, TX
Barring GOP landslide in either 2018 or 2020, I fail to see how the Senate isn't at least 50-50 in 2021.
Republicans don't need a landslide in 2018 to win MO (lol), IN and one or two of OH/ND/MT/WV/PA.
More than 1 or 2. They are gonna get Blanched.
Remember when Mark Pryor thought he was tough stuff because he was polling close to Cotton? Lol. Blanched!