Currently the Dems have 47 senators (45Ds + 2Is) the best case scenario of Dems Senate Wise in 2020 is 52 senators and that is an extremely hopeful case. And most likely the Democrats are going to lose the Alabama Doug Jones seat.
So let's say 52 senate seats happen. Good Luck getting Joe Manchin to go with the most extreme Democratic agenda. And depending on the issue for things like farming and so on it will be quite easy to lose 2 other seats.
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Real talk here, it is not going to be 52 senate seats come 2020 / 2021.