Joe Manchin because his replacement would flip the Senate to GOP control.
That would be stupid since he's basically already the senator the GOP can convince to do things while other Democrats aren't open, and he's already DOA.
For me personally, there are a number of options.
For GOP, you can argue the majority could even be expelled for violating the rule of democracy, in regards to not condemning the Trump riots. I even don't know who I would expel, and i don't know if it would make sense since the replacement would possibly be as bad, possibly even much worse. The GOP senators that can be replaced, could easily be democratically removed from office if Democrats run a good campaign.
I'd choose to expel Mitch McConnell since he's a big reason why the GOP is moving into the direction that we don't want it to move, and since he fails to condemns it.For the Democrats, if i'd remove a Democrat it would be easier. Dianne Feinstein is the first senator I would replace, but I would bet that she would die or be removed from office soon regardless of expulsion, so i'd save it for Bob Menendez or Kyrsten Sinema. Since I doubt their replacements will be better and I had to expel a democrat,
i would probably expel Kirsten Gillibrand, since she seems a grifter too (throwing Franken under the bus, being rightfully critical of Bill Clinton but failing to condemn Cuomo) and we can get EASILY someone better from New York, guaranteed to be a Democrat.
