The office of U.S. Senator Tim Johnson says the South Dakota Democrat's condition has been upgraded from critical to fair, after he was hospitalized for a brain hemorrhage four weeks ago.

Johnson's office said Tuesday that the senator may begin rehabilitation this week and that his recovery is expected to take several months.

Johnson underwent emergency surgery in December for bleeding in his brain.

Democrats, along with two independent senators expected to vote with them, currently hold a 51-49 majority in the U.S. Senate.  If Johnson were unable to serve out his term, South Dakota's governor could appoint a fellow Republican to the position.  This would result in a 50-50 split in the Senate, with Vice President Dick Cheney casting any tie-breaking votes, thereby tipping the balance of power over to Republicans.

Some information for this report was provided by AP.