[mrsmith]

"Your assumption here is that Johnson wouldn't be removed because of incapacitation, leaving SD with in effect, 1 senator. I highly doubt that would stand for very long. "

Except for death, the only way a Senator can be removed is resignation or expulsion.

No one can make him resign and the Dems can't be forced to expel him so tough nuggies for S.D.

I believe (haven't found a source) that the leadership remains in place throughout a Senate session even if the majority changes. If that is so then the Dems will expel him after the leadership vote if he is incapacitated. Then SD could replace him.

[STARWISE]

This article claims incapacitation can't force stepping down:

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There is little precedent for forcing a living senator to stand down against his will.

A predecessor of Mr Johnson, Karl Mundt, continued to hold his Senate seat for three years after a stroke that incapacitated him in 1969, although he was unable to attend Senate sessions.