Sen. Tim Johnson, a Democrat from South Dakota, was hospitalized Wednesday after what his office at first called a "possible stroke," leading to concern among Democrats that their new one-vote majority in the Senate could be in danger. By the end of the day, after what his office called "a comprehensive evaluation by the stroke team" at George Washington University Hospital here, a spokeswoman for Johnson said that he had not suffered a stroke or a heart attack. The spokeswoman, Julianne Fisher, gave no further details about Johnson's medical condition.His office said that Johnson, who will turn 60 on Dec....