Story Highlights • NEW: Staff says tests rule out stroke, heart attack

• Senator undergoing more tests, will remain hospitalized overnight, aide says

• If Democrat Johnson could not serve, GOP governor could name replacement

• Johnson has been a South Dakota senator since 1996

Adjust font size:

WASHINGTON (CNN) -- Sen. Tim Johnson, D-South Dakota, was hospitalized Wednesday after he suffered stroke-like symptoms in his Washington office, his staff said.

Johnson, who turns 60 on December 28, was taken to George Washington University Hospital by ambulance about 11:30 a.m., sources in his office said.

A statement issued by Johnson's office said he was suffering from a "possible stroke." But a spokeswoman later said that tests showed Johnson had suffered neither a stroke nor a heart attack, although it was not clear what had happened. (Listen to Johnson's difficulty speaking during a WNAX radio interview -- MP3, 749 kb)

Julianne Fisher, a spokeswoman for Johnson, told CNN he was still undergoing tests and will stay hospitalized overnight, but she would not characterize his condition.

"We're taking one day at a time," Fisher said, "saying a lot of prayers and waiting until morning."

Staff members said that Johnson was conscious when he was taken to the hospital.

Fisher said Johnson was in the Capitol Wednesday morning conducting a conference call with South Dakota reporters when "his speech pattern slipped off."

She said the senator was able to walk back to his office in the Hart Senate Office Building, then began having problems with his right arm. He thought he was all right, she said, and went to his desk, but came out a few minutes later and "it was apparent he needed help."

Staffers put him on a couch in the office and called the Capitol doctor, she said. He was taken to the hospital shortly afterward. His wife, in the office to have lunch with him, rode with him, Fisher said.

A lawyer and longtime state lawmaker, Johnson was first elected to the U.S. House of Representatives in 1986. He served five terms before he was elected to the Senate in 1996.

He is the senior senator from South Dakota and serves on numerous committees, including appropriations, budget, banking, energy and natural resources, and Indian affairs.

Should Johnson not be able to complete his term, which ends in 2008, South Dakota Gov. Mike Rounds, a Republican, could appoint his replacement, which could shift the balance of power in the Senate.

South Dakota Secretary of State Chris Nelson said the appointment would fill the vacancy until a general election could be held in November 2008. There are no restrictions on who the governor can appoint, beyond meeting the legal requirements for Senate membership, he said.

Johnson battled prostate cancer in 2004, and after surgery, tests showed he no longer had the disease, according to his Web site.