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

Mr. Warner, 80, has held the seat since 1979, when the dashing former Navy secretary campaigned alongside his wife at the time, Elizabeth Taylor.

Mr. 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.

“Public service is a privilege and I urge all to try to find time some time in their lifetime to serve the needs of others,” Mr. Warner said at a news conference. “I have tried to be not only true to myself, but true to the people of this great commonwealth I’ve served for 29 years now.”

Mr. Warner, who chaired the Armed Services Committee when the GOP controlled the Senate, thanked those who had helped give him “a magnificent and very rewarding career” in the Senate.

Mr. Warner, a courtly senator with chiseled features and a full shock of gray hair, chose a sentimental setting for his announcement: the steps of the University of Virginia’s signature structure, the Rotunda, designed by school founder Thomas Jefferson.

The most likely Democratic contender is a former Governor, Mark Warner, a wealthy 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 probably will include a former Governor, Jim Gilmore, and Rep. Thomas M. Davis.

Last week, Mr. Warner returned from a trip to Iraq and rocked the Republican White House with a call for President Bush to begin a troop withdrawal by Christmas.

The GOP nominated Mr. Warner for the Senate in 1978 after the party’s first choice, Richard Obenshain, was killed in a plane crash. Mr. Warner was elected by the razor-thin margin of 4,721 votes out of 1.2 million cast. He was easily re-elected in 1984 and 1990. He beat Mark Warner by about 5 percentage points in 1996 and was unopposed in 2002.

In 1996, the conservative wing of his own party tried to deny Mr. Warner a fourth term. Mr. Warner had angered conservatives two years earlier by opposing GOP nominee Oliver North’s bid to unseat Democratic Senator Robb. Mr. Warner declared the Iran-Contra figure unfit for public office and backed an independent who drew enough votes from moderates to ensure Mr. Robb’s re-election.

Angered by what they viewed as party disloyalty, GOP conservatives backed a former Reagan administration budget director, Jim Miller, to challenge Mr. Warner for renomination. Mr. Warner easily defeated Mr. Miller in a primary.

Mr. Warner mended his relationship with the GOP by supporting the successful campaigns of Mr. Gilmore for governor in 1997 and George Allen for Mr. Robb’s Senate seat in 2000.

Governor Kaine, a Democrat, called Mr. Warner “a good friend, a great Virginian, and a true statesman.”

“All Virginians should honor his distinguished leadership,” Mr. Kaine said.

Mr. Warner’s actions in recent weeks had fueled speculation that he would retire. Several longtime staff aides and advisers moved on to other jobs, and he raised less than $75,000 in campaign funds for the first half of 2007, including only $500 in the first three months.