See this from Bob Novak:
TIM JOHNSON'S SEAT
National Democratic Party strategists believe that Sen. Tim Johnson, out of sight since suffering a brain hemorrhage last Dec. 13, will be able to run for re-election in South Dakota next year. But if he cannot, they fear Democrats will lose the seat in the heavily Republican state.
These strategists doubt Rep. Stephanie Herseth Sandlin, the probable Democratic nominee if Johnson does not run, can win a Senate race. The strongest Republican candidate would be Gov. Mike Rounds. Other possibilities are Lt. Gov. Steve Kirby and State Senate Majority Leader Dave Knudson.
South Dakota Republican insiders say they doubt Johnson will be able to be a candidate. His office has said he may return to the Senate as early as this summer, with the help of a wheelchair.