The Democrats' new majority in the Senate could be threatened by a senator's sudden illness. Senator Tim Johnson of South Dakota was hospitalized with the symptoms of a stroke Wednesday and underwent surgery several hours later. There was no word on the nature of the surgery, which lasted into early Thursday, or on 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 Johnson is forced to relinquish his seat, a replacement would be named by the Republican governor of South Dakota to finish his term, which runs through 2008. A Republican appointee would create at 50-50 tie, allowing the Republicans to retain Senate control. If Senate voting is split 50-50, the vice president - currently Republican Dick Cheney - acts as the tie-breaker. 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, 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. 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 Wednesday, 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 Johnson 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." Democratic Senate leader Harry Reid 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. Johnson, a centrist Democrat, was first elected to the Senate in 1996 and has been one of the more reserved members of the chamber, rarely taking center stage at news conferences. He served in the House of Representatives for 10 years from 1987 to 1997. The senator underwent prostate cancer treatment in 2004, and subsequent tests have shown him to be clear of the disease. His wife, Barb, is a breast cancer survivor. The couple have three adult children. Johnson is the second senator to become ill after the Nov. 7 election. Wyoming Senator Craig Thomas, a Republican, was diagnosed with leukemia on Election Day. He is back at work.