WASHINGTON, Dec. 13 (UPI) -- Sen. Tim Johnson, D-S.D., was hospitalized in Washington Wednesday, but his office said he did not suffer a stroke or a heart attack, as earlier thought.
Johnson, 59, underwent "a comprehensive evaluation by the stroke team" at George Washington University Hospital, a statement from his office said.
Spokeswoman Julianne Fisher told the Sioux Falls (S.D.) Argus Leader Johnson was admitted to GWU Hospital with an undiagnosed illness.
His condition was not released.
Rep. Stephanie Herseth, D-S.D., had earlier said she believed Johnson had suffered a severe stroke, the newspaper said.
South Dakota Gov. Mike Rounds, a Republican, would appoint Johnson's successor if Johnson is unable to serve when the 110th Congress convenes in January. One result would be a loss of control of the Senate by Democrats, since Rounds would be expected to appoint a Republican to replace Johnson.
Democrats won a one-seat Senate majority in the November midterm elections, but a loss of Johnson's seat to Republicans would split the Senate 50-50 -- and effectively give control of the chamber to the GOP, since Vice President Dick Cheney would cast any tie-breaking votes.
Johnson was elected to the Senate in 1996 when he defeated Republican Sen. Larry Pressler. He won an extremely competitive race in 2002 against Republican Rep. John Thune, whom he defeated by 524 votes, the Washington Post reported.
Johnson is up for re-election in 2008.