S.D. Sen. Tim Johnson Undergoing Surgery

Sen. Tim Johnson, D-S.D., chats during a meeting in Pierre, S.D. in this Jan. 11, 2006 file photo. Johnson was taken to a George Washington University Hospital in Washington, Wednesday, Dec. 13, 2006 after suffering a possible stroke. (AP Photo/Jenny Michael, File) (Jenny Michael - AP)

WASHINGTON -- Democratic Sen. Tim Johnson of South Dakota was undergoing surgery early Thursday after being admitted to the hospital with symptoms of a stroke weeks before his party is slated to take control of the Senate by a one-vote margin.

There was no word on the nature of the surgery _ which began late Wednesday and lasted past midnight _ or Johnson's condition.

Apart from the risk to his health, Johnson's illness carried political ramifications. Democrats emerged from last month's elections with a 51-49 Senate majority. If he is forced to relinquish his seat, a replacement would be named by South Dakota's GOP Gov. Mike Rounds.

A Republican appointee would create at 50-50 tie, and allow the GOP to retain Senate control.

Johnson, who turns 60 later this month, was admitted to George Washington University hospital at midday Wednesday after experiencing what his office initially said was a possible stroke.

His spokeswoman, Julianne Fisher, later told reporters it had been determined that the senator had suffered neither a stroke nor a heart attack.

But several hours after she spoke, Dr. John Eisold, the Capitol physician, called that into question with a statement of his own.

"Senator Tim Johnson was admitted to the George Washington University Hospital today with the symptoms of a stroke. He is currently under the care of physicians at the George Washington University Hospital."

Fisher and Eisold both said they did not intend to issue further updates on Johnson's condition until Thursday. There was no formal announcement of the surgery, which was disclosed by an official who spoke on condition of anonymity, citing the sensitive nature of the subject.

Johnson became disoriented during a conference call with reporters, stuttering in response to a question.

Before he ended the call, Johnson appeared to recover and asked if there were any additional questions.

Fisher said he then walked back to his Capitol office but appeared to not be feeling well. The Capitol physician came to his office and examined him, and it was decided he should go to the hospital.