Aug. 31 (Bloomberg) -- Virginia Republican John Warner, one of his party's most prominent figures on defense issues, won't seek re-election to the U.S. Senate, boosting Democratic chances of capturing the seat next year.

``I've done my best,'' Warner, 80, said at a press conference in Charlottesville. ``I'm quietly going to step aside and clear the way for others.''

Warner, a former secretary of the Navy and chairman of the Senate Armed Service Committee, was an early supporter of the Iraq war. This month, after a visit to Iraq, Warner called on President George W. Bush to begin withdrawing troops to show the Iraqi government that the U.S. commitment is not open-ended.

``I feel I have played, I am playing, and I will continue to play a role in trying to bring about a cessation of that conflict,'' he said today.

Replacing Warner next year will set off a heated battle as Republicans seek to overcome the Democrats' 51-49 majority control of the Senate.

While Bush carried Virginia in the 2004 presidential election, the state's other Republican senator, George Allen, was defeated by Democrat Jim Webb in the 2006 congressional elections. Governor Tim Kaine, who was elected in 2005, is also a Democrat.

`Difficult to Beat'

While Warner ``would have been very difficult to beat,'' his absence gives Democrats an opportunity, said Carl Tobias, a professor at the University of Richmond School of Law. ``The Democrats have been quite successful in these recent statewide elections,'' he said.

Former Governor James Gilmore and Representative Tom Davis are among the Republicans who may run for the office. Former Governor Mark Warner, who is not related, may be a Democratic candidate.

Tobias said Mark Warner would be an obvious front-runner for the seat, should he decide to run.

Both Davis and Warner issued statements praising Senator Warner, while declining to comment on their plans.

Nevada Senator John Ensign, chairman of the National Republican Senatorial Committee, said the group ``will be working with Senator Warner to field a strong Republican candidate to continue his work on behalf of the people of Virginia.''

To contact the reporter on this story: Nicholas Johnston in Washington at njohnston3@bloomberg.net.

Last Updated: August 31, 2007 16:18 EDT