Sen. Tim Johnson remains in the hospital, but early suspicions of a stroke have been ruled out. A spokeswoman for the senator says doctors have ruled out either a heart attack or a stroke as the reason why the senator became ill.
Earlier Wednesday, it was believed that Johnson, a Democrat from South Dakota, may have suffered a stroke.
The two-term senator, who turns 60 later this month, was on a conference call talking with reporters when he had difficulty speaking. He finished the conversation, but he complained of feeling ill. Soon after, he was taken to George Washington Hospital in Washington, D.C.
Reports of Johnson's condition immediately set off speculation about what would happen should he be incapacitated.
If Johnson is unable to serve, control of the Senate could potentially stay with the Republicans for the 110th Congress.
Under South Dakota law, if Johnson is unable to serve his term, Gov. Mike Rounds, a Republican, would appoint a temporary replacement — likely a Republican. If needed, the appointment would leave the Senate tied 50-50, with Vice President Cheney delivering the tie-breaking vote.