CHARLOTTESVILLE, Va. Republican Sen. John Warner of Virginia, one of the most authoritative voices in Congress on the military and a key figure in the debate over Iraq, said Friday he will not seek a sixth term in 2008.

Warner, 80, a former Navy secretary, was elected in 1978, after campaigning alongside his wife at the time, Elizabeth Taylor.

Warner is leaving what would have been a safe seat for the Republicans if he had chosen to run again. His departure gives Democrats a better chance to protect or even expand their one-seat majority in the Senate.

Warner said he still feels spry enough for the rigors of the Senate, but he did not want to push it for another six years. “I want to be fair to this wonderful state, which has been eminently fair to me all these years,” Warner said.

His message to Virginians, he said, is: “You’ve given me my best shot, and I’m quietly going to step aside and make way for others.”

Warner, former chairman of the Senate Armed Services Committee, has long been an important voice in the Iraq debate because of his military expertise and his willingness to question White House war policy.

After a recent trip to Iraq, Warner said President Bush should start bringing some troops home by Christmas. If Bush refused, Warner said, he would consider backing anti-war legislation. The statement was the senator’s strongest to oppose the White House and is expected to influence the Iraq debate this fall.

Bush on Friday called Warner “one of the most dedicated senators in American history.”

Sen. Hillary Clinton said she will miss her “friend and colleague. At a time when the tone in Washington is so often defined by partisanship and rancor, Senator Warner has always risen above the fray,” Clinton said.

Warner is Virginia’s second-longest serving senator, with a tenure surpassed only by that of Sen. Harry Byrd, who held office from 1933 to 1965.

The most likely Democratic contender is former Gov. Mark Warner, an Alexandria businessman who left office in early 2006 with unprecedented job-approval ratings. Mark Warner, who is not related to the senator, unsuccessfully challenged him in the 1996 Senate race.

The nomination battle among Republicans will probably include former Gov. Jim Gilmore and U.S. Rep. Thomas Davis.