S.D. Sen. Tim Johnson Undergoing Surgery

Sen. Tim Johnson, D-S.D., chats during a meeting in Pierre, S.D. in this Jan. 11, 2006 file photo. Johnson was taken to a George Washington University Hospital in Washington, Wednesday, Dec. 13, 2006 after suffering a possible stroke. (AP Photo/Jenny Michael, File) (Jenny Michael - AP)

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 of Nevada 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.

If the Senate is split 50-50, the vice president breaks ties. That is Republican Dick Cheney at present. Johnson is up for re-election in 2008.

South Dakota Secretary of State Chris Nelson said there are no special restrictions on such an appointment by the governor and a replacement would not have to be from the same political party.

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 for 10 years from 1987 to 1997. His focus has been on committee assignments important to his state's interests _ Indian Affairs and Energy and Natural Resources _ as well as a spot on Appropriations. The latter allows him to direct funds to South Dakota.

Johnson narrowly defeated Republican John Thune in his 2002 re-election bid. Thune defeated Sen. Tom Daschle, the former Senate Democratic leader, two years later.

Johnson has worked as a lawyer and county prosecutor and served several years in the 1970s and 1980s in the South Dakota state Legislature.

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.

In response to one reporter's question on the conference call Wednesday, Johnson said he was most looking forward to spending the holidays with his family and grandchildren.

Johnson is the second senator to become ill after the Nov. 7 election. Wyoming Sen. Craig Thomas, a Republican, was diagnosed with leukemia on Election Day. He is back at work.

____

Associated Press writers Laurie Kellman and Natasha Metzler in Washington and Dirk Lammers in Sioux Falls contributed to this report.