South Dakota Senator Tim Johnson has apparently suffered a stroke. Johnson is a Democrat, which means, according to the Washington Post:
Should Johnson be unable to serve when the 110th Congress convenes in January, it could mean a 50-50 split in the Senate. Gov. Mike Rounds (R) would be tasked with appointing a successor to Johnson -- presumably a Republican. That could effectively put the Senate, which is slated to switch to Democratic control in January, in Republican hands because Vice President Cheney would cast the tie-breaking vote.