Sen. Tim Johnson

First, I hope and pray that the man has a full and speedy recovery. Secondly I'm taking bets on how long it will be before the tin foil hat crowd start the conspiracy murmurings. I'm betting that it has already happened, any takers?
What's at stake? If Sen. Johnson can't return to work the US Senate is back to 50/50:

Johnson's illness -- which sent Senate Democratic leader Harry M. Reid (Nev.) rushing to the hospital to check on Johnson -- underscored the fragility of democrats' hold on the next Senate, which they won by the narrowest of margins in the Nov. 7 elections. Should Johnson be unable to complete his term, South Dakota's Republican governor, Michael Rounds, would name a replacement for the next two years.
With Johnson in office, Democrats would hold a 51-to-49 edge in the Senate that convenes Jan. 4 as part of the 110th Congress. (The two independents have said they will caucus with the Democrats.) But if he is to leave office before then and Rounds replaces him with a Republican, the GOP would control the chamber.

Karl Rove's mind beams are all powerful and not to be trifled with.