WASHINGTON -- U.S. Sen. Fred Thompson, R-Tennessee, has abruptly announced he will retire when his term expires in January, complicating GOP efforts to recapture Senate control in the fall elections.

Thompson, familiar to millions as Republican counsel to the 1973 Senate Watergate Committee and an actor in more than a dozen movies, had mulled retirement last year. He announced two weeks after the Sept. 11 terrorist attacks that he would run again because "now is clearly not the time to leave."

But colleagues said he was hit hard by the January death of his daughter, Elizabeth Thompson Panici, 38, following a heart attack. Thompson, 59, who has two other children, is divorced.

"I simply do not have the heart for another six-year term," Thompson said in a written statement. "Serving in the Senate has been a tremendous honor, but I feel that I have other priorities that I need to attend to."

The decision seemed to end -- at least for now -- a political career in which he had been seen as a rising Republican star and possible 2000 running mate for George W. Bush.

It also set off a scramble to replace him in a state that has leaned Republican in recent elections.

Al Gore, the 2000 Democratic presidential candidate and former vice president, said he would not seek the Senate seat he once held.

GOP sources said former U.S. Education Secretary and Tennessee Gov. Lamar Alexander, a Republican, would announce on Monday that he is running. Aides said Democratic Reps. Harold E. Ford Jr., Bob Clement and Bart Gordon and Republican Reps. Ed Bryant and Zach Wamp were each pondering a challenge.

Thompson's departure brings to four the number of GOP senators who are leaving the chamber in January.