Last week Senator Tim Johnson (D-SD) suffered what appeared to be a stroke. He lost his ability to speak. After the Attending Physician to the Congress, Rear Admiral John Eisold, MC, USN, examined him he was rushed to George Washington University Hospital, where he underwent brain surgery. It seems he was bleeding from blood vessels which never had formed properly. Johnson has an unusual birth defect, apparently termed arteriovenous malformation, estimated to affect some 300,000 Americans. Now doctors report he is recovering but the long-term prognosis is much harder to predict.
The first person to rush to Johnson's side was Majority Leader-elect Harry M. Reid (D-NV). I am sure it occurred to Reid that should the Johnson Senate seat be vacated the Republican Governor of South Dakota no doubt would appoint a Republican to serve until the next election. That would move the Senate to a 50-50 tie.
Inasmuch as Vice President Richard B. Cheney breaks ties, the Senate would be in Republican hands. That, in fact, was the situation in 2000. There was a tie and Cheney broke it as soon as he was sworn into office. From that day the Republicans were in control. Then when Vermont Senator James M. Jeffords caucused with the Democrats, after he became a so-called Independent, Republicans lost control.
Senator Reid does not have to worry about his precarious majority unless Senator Johnson were to die or to resign.
There is a precedent in Johnson's own State which probably would deter him from resigning regardless of his condition.
In 1966 Senator Karl Mundt handily was re-elected. He served until November 1969, when he suffered a stroke, which left him in a virtual vegetative state.