(CNN) -- Sen. John Warner, R-Virginia -- the respected former Navy secretary and outspoken critic of the current state of affairs in Iraq -- will not seek re-election to a sixth term, he announced Friday.
Sen. John Warner, R-Virginia, was first elected to his seat in 1978.
"I say that my work and service to Virginia as a senator will conclude upon the sixth of January, 2009," said Warner, 80, during a news conference in Charlottesville, Virginia, at the University of Virginia, his alma mater.
"Now, by taking this action, no one can say politics is going to dictate in one way or another how I'm going to decide to speak out on what's in the best interests of this nation. And I'm going to do that," Warner said, noting that some have read his recent public criticisms of Iraqi leaders as political posturing.
Ending his Senate career after 30 years, he said, he will be "the second-longest serving United States senator in the history of the Commonwealth of Virginia."
Though Warner recently rankled the Bush administration with his call for the president to withdraw some U.S. troops from Iraq by Christmas, Bush praised him in a written statement.
"With Sen. Warner's retirement, the Senate will lose one of its most independent and widely respected voices and the Commonwealth of Virginia will lose one of its fiercest advocates," he said. Watch Warner talk about his decision and Iraq with CNN's Wolf Blitzer »
Warner's retirement puts Senate Republicans in a bigger bind as they try to recapture the chamber from Democrats in next year's election. Watch Warner's announcement
Virginia was once a solid red state, but Democrats have won most of the major statewide elections in recent years.
In a phone conversation with CNN senior political correspondent Candy Crowley, John Warner said he had "mixed emotions" about his retirement. He admitted that worry over whether his seat might be won by a Democrat in 2008 was a factor in his decision, but indicated his age may have played a bigger role. Watch Candy Crowley discuss the impact of Warner's decision
Warner, who would be almost 82 at the start of a sixth term, called his Senate job "so dynamic, so demanding that it is really 24-7."
"In the end, my thinking was driven by one thing, by what is fair, not to me, but to the people of this state," Warner said. He acknowledged his retirement put Virginia in play for Democrats, but said "you take the cards the voters deal you and go for it."
Rep. Tom Davis appears to be the first person to throw his hat into the ring in the battle to succeed Warner.
"Tom's running. He didn't want to say more today because it's the senator's day," a congressional source close to Davis told CNN Radio's Lisa Goddard.
Davis, a moderate Republican from Northern Virginia, has long eyed succeeding Warner. But he could most likely face serious competition from a conservative candidate. One possibility is former Virginia Gov. Jim Gilmore, who gave up his bid for the White House last month.
On the Democratic side, party leaders are hoping former Gov. Mark Warner (no relation) will run. Mark Warner left office in 2005 with very favorable ratings. He also considered running for president but never officially jumped into the race. The former governor would be considered a very strong candidate and could have a good chance at taking the seat away from Republicans.
Mark Warner will make his intentions known next week, said his spokeswoman, Monica Dixon.
On Iraq, John Warner told Crowley the war is not lost, saying there have been "some measurable gains."
He added that American soldiers are helping sustain the Iraqi government by giving it "breathing space, so the legislative body can do their work." But he said Iraqi Prime Minister Nuri al-Maliki's government is not living up to its end of the bargain.
Warner opposed Bush's January decision to send nearly 30,000 additional U.S. troops to Iraq. But he has so far also opposed Democratic efforts to force Bush to start bringing U.S. troops home.
He and the current Armed Services chairman, Sen. Carl Levin, D-Michigan, recently returned from a visit to Baghdad with harsh words for al-Maliki's government.
Levin said last week that Iraq's parliament should throw al-Maliki out of office and replace his government. Warner said he would not join that call. "But in no way do I criticize it," he added.
When he retires, Warner will leave behind a strong legacy. He's one of the most respected voices in the Senate on military and national security issues.
Warner served in the Navy during World War II, enlisting at the age of 17, according to his Web site. He also was in the Korean War as a U.S. Marine. He worked in the Department of the Navy for five years during the Vietnam War and was secretary of the Navy from 1972 to 1974.
Warner was first elected to the Senate in 1978 and has been re-elected four times. In addition to the Armed Services Committee, he is on the Intelligence Committee, the Committee on the Environment and Public Works, and the Committee on Homeland Security and Government Affairs.
According to the most recent filing with the Federal Election Commission, Warner's campaign committee had $734,494 on hand as of June 30 and had raised a little more than $72,000 in the second quarter of 2007.
Warner has endorsed Sen. John McCain of Arizona in the 2008 race for the Republican presidential nomination. E-mail to a friend
CNN's Candy Crowley, Lisa Goddard and Paul Steinhauser contributed to this report.
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