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."
"At this stage he is undergoing a comprehensive evaluation by the stroke team," the statement said. Staffers said that Johnson was conscious when he was transported to the hospital.
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.
Johnson battled prostate cancer in 2004, and after surgery, tests showed he no longer had the disease, according to his Web site.