Sorry to hear that he’s suffering this level of disability, but I’m sure Harry Reid will assign a permanent staffer to make his votes for him, just in case he gets “confused”.
I don't know the political make-up of S. Dakota, whether red or blue, but, if a senator becomes unable to perform in his office ... isn't it then up to the governor to appoint a senator to replace him, and isn't it usually the party of the governor?
When is Sen. Johnson's term up?
I'll go and try to answer my own questions, but FreepeRs are usually quicker and more exact than I.