Sen. Tim Johnson's condition has been upgraded from critical to fair, four weeks after he was hospitalized for a brain hemorrhage, his office said Tuesday.

The South Dakota Democrat, who was rushed to the hospital on Dec. 13 and underwent emergency surgery, remains in intensive care, said his spokeswoman, Julianne Fisher.

"The senator continues to make progress," Fisher said. "The next step would be rehabilitation and we hope that would happen within the week."

Johnson's office has said that his recovery is expected to take several months. He underwent surgery to correct a condition called arteriovenous malformation, involving tangled arteries in his brain.

The senator's doctors said last week that Johnson was improving but still needed a ventilator at night to help him breathe. The ventilator has required a tube to be placed down Johnson's throat, making it impossible for him to talk.

His long-term prognosis is unclear. He has been responsive to his family and physicians, following commands, squeezing his wife's hand and understanding speech.

The senator's sudden illness raised questions about the Democrats' one-vote majority in the upcoming Senate session. South Dakota's Republican governor, Mike Rounds, would appoint a replacement if Johnson's seat were vacated by his death or resignation.

A Republican appointee would create a 50-50 tie and could effectively allow the GOP to retain Senate control because of Vice President Dick Cheney's tie-breaking vote.