Democratic Sen. Tim Johnson was in critical condition early Thursday, recovering from emergency surgery overnight, a hospital official said.
Johnson suffered stroke-like symptoms Wednesday, just weeks before his party, with only a one-vote majority, was to take control of the Senate.
There was no formal announcement of the South Dakota senator's condition. A person in the hospital's media relations office, who declined to be identified by name pending a formal statement, said George Washington University Hospital was preparing to announce that Johnson's condition was critical. He would not describe the nature of the surgery.
There was no formal announcement of the operation, which lasted past midnight Wednesday, and was disclosed by another official who spoke on condition of anonymity, citing the sensitive nature of the subject.
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 were forced to relinquish his seat, a replacement would be named by South Dakota's GOP Gov. Mike Rounds.
A Republican appointee would create a 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 after experiencing what his office initially said was a possible stroke.
His spokeswoman, Julianne Fisher, told reporters that 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.
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 whether 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.
He was taken to the hospital by ambulance around noon, Fisher said.
"It was caught very early," she said.
In its earlier statement, Johnson's office had said he had suffered a possible stroke and was "undergoing a comprehensive evaluation by the stroke team."
Senate Democratic leader Harry Reid of Nevada went to the hospital to check on Johnson. He called Johnson a "dear friend to me and to all of us here in the Senate."
The White House also issued a statement wishing him a speedy recovery. "Our thoughts and prayers are with Senator Johnson and his family," said spokesman Alex Conant.
If the Senate is split 50-50, the vice president breaks ties. That is Republican Dick Cheney at present. Johnson is up for re-election in 2008.
South Dakota Secretary of State Chris Nelson said there were no special restrictions on such an appointment by the governor and a replacement would not have to be from the same political party.
Johnson, a centrist Democrat, was first elected to the Senate in 1996 after serving 10 years in the House. He narrowly defeated Republican John Thune in his 2002 re-election bid. Thune defeated Sen. Tom Daschle, the former Senate Democratic leader, two years later.
Johnson underwent prostate cancer treatment in 2004, and subsequent tests have shown him to be clear of the disease.
Johnson is the second senator to become ill after the Nov. 7 election. Wyoming Sen. Craig Thomas, a Republican, was diagnosed with leukemia on Election Day. He is back at work.