Senator Tim Johnson:

First, best wishes for a speedy and complete recovery to Sen. Johnson (D-SD).

Second, I've heard someone ask whether — if Sen. Johnson unfortunately proves to be incapacitated — the current Senate majority could remove him from office under the Constitution's provision that "[e]ach House shall be the Judge of the Elections, Returns and Qualifications of its own Members" (art. I, cl. 5, § 1). The answer is no, under Powell v. McCormack, 395 U.S. 486 (1969): "[I]n judging the qualifications of its members Congress is limited to the standing qualifications prescribed in the Constitution," namely "age, citizenship, and residence." In judging the elections and returns each House is of course also allowed to decide whether the person got a majority of the vote. But the clause does not authorize exclusion for misconduct (the next clause, which provides that "Each House may determine the Rules of its Proceedings, punish its Members for disorderly Behaviour, and, with the Concurrence of two thirds, expel a Member," does so, though it requires a 2/3 vote rather than a majority) or for perceived incapacity.

(Thanks to Hugh Brady for pointing this out.)