Sen. Tim Johnson (D-S.D.) underwent emergency brain surgery overnight after falling ill at the Capitol and was in critical condition early this morning, introducing a note of uncertainty over control of the Senate just weeks before Democrats are to take over with a one-vote margin. Johnson, 59, was taken to George Washington University Hospital early yesterday afternoon, shortly after becoming disoriented during a conference call with news reporters. He underwent "a comprehensive evaluation by the stroke team," his office said. Aides later said he had not suffered a stroke or heart attack.