Cite? The Constitution is silent on this. It gives the Senate the power to expel a member, but the clause is ambiguous as to whether this is a universal power, or only for punishing misconduct. If universal, then it follows that any Party which gains 67 seats could arbitrarily expel the opposition. It gives the Senate the power to refuse to seat a replacement.
The only references to a vacancy are "if Vacancies happen by Resignation, or otherwise,..." that the State gets to appoint a new Senator, and then the language in the 17th A. All S.D. needs to do is file suit saying that this is a case of vacancy by "otherwise" than resignation: i.e. incapacity.
Let me restate my comment a bit. Other than in the case of a death Congress decides who will be seated. And, yes, it theoretically could refuse to seat a winner. We may see this in a Florida race with the RATS refusing to seat a Republican House seat winner. There is NO vacancy without a resignation or death. Rather the Senate would have to decide that Johnson is "qualified" to be a Senator.
So it could decide that a comatose Johnson is qualified.
I see no constitutional power to force the Senate to do anything wrt a member. And it could expel the opposition with 67 votes however, it would have to keep doing this as the States affected would immediately re-appoint the same or another Senator.
Adam Clayton Powell was thrown out of the House and was quickly re-elected by his Harlem district. Eventually the House knuckled under and seated him.