President Bush’s vow to keep U.S. troops in Iraq beyond his term in office did more than underscore his determination to see the controversial mission through to a successful ending.

It ensured that the question of when American forces can come home would be a major issue in the 2008 election and could turn it into a referendum on his policy in Iraq, just as the 1952 and 1968 elections revolved around the controversial decisions of two prior presidents to send U.S. troops to Korea and Vietnam.

Although national security is widely regarded as the Republican Party’s ultimate trump card, history reminds us that, in both instances, dissatisfaction with those wars was a major factor in the decision by voters to throw the incumbent party out of office.

Iraq probably always was destined to be a major issue in 2008. But the greater the focus on it and other international issues, the tougher it may be for candidates who lack foreign policy experience – such as Republican Govs. Mitt Romney of Massachusetts and Mike Huckabee of Arkansas, and Democratic Gov. Tom Vilsack of Iowa and former Gov. Mark Warner of Virginia.

And given decreasing domestic support for the war, the fact that some U.S. forces will still be there could complicate the candidacies of the early front-runners, Republican Sen. John McCain and Democratic Sen. Hillary Rodham Clinton.

Both backed Bush’s decision to use force to overthrow Saddam Hussein, though both have criticized how he has done it.

McCain has pressed Bush to send more troops to stamp out the insurgency and pave the way for the democratic Iraq that the president vows to achieve.

Of all the major contenders, he has probably been the most outspoken in defending the president on the war, which serves to balance his criticism of other Bush policies, such as tax cuts.

Ironically, however, that could put the Arizona senator and onetime Vietnam prisoner of war on the defensive if domestic pressure for a U.S. pullout increases between now and 2008.

Most other GOP hopefuls have also supported the president on the war, except for Sen. Chuck Hagel of Nebraska, a Republican maverick who most analysts believe is unlikely to enter the race unless Mr. McCain doesn’t run.

Potential Democratic candidates have more varied stances.

The president’s strongest Democratic ally, Sen. Joe Lieberman of Connecticut, is unlikely to run. But the field likely will include some sharp critics and some who backed the attack but have criticized how the policy has been pursued.

Two of the main factors in determining the 2008 political dynamic may be the state of affairs in Iraq and the degree to which Democratic primary and caucus voters back candidates who campaign on the basis of getting troops out sooner rather than later.

That group could include not only Sen. Russell Feingold of Wisconsin, a consistent war foe, but some who initially backed it and have become critical, such as the men who formed the party’s 2004 ticket, Sen. John Kerry of Massachusetts and former Sen. John Edwards of North Carolina.

Of those who have supported the war, the two most likely contenders are Clinton and Sen. Joseph Biden of Delaware. They would face the trickiest task in 2008, having to justify past support for an unpopular war in a party where the majority is on the other side while maintaining a stance opposed to abandoning the effort.

In 1968, both general election candidates, Republican Richard Nixon and Democrat Hubert Humphrey, had backed the Vietnam War. Humphrey sought to break with President Lyndon Johnson’s unpopular war policy in the campaign, satisfying neither side.

Nixon, with more freedom to maneuver as the out-of-power candidate, won, in part by persuading voters he had an unspecified plan to end the war. It turned out to be gradual withdrawal, which took another four years.

In 1952, the debate and the election were also won by the candidate of the party out of power. Republican Dwight Eisenhower had the advantage of being a war hero. Along with his promise to go to Korea to study the situation, that carried the day.

If history repeats itself, the 2008 winner may be whoever is able to persuade voters he can end the U.S. involvement in Iraq while achieving at least some of the goal that prompted Mr. Bush to attack in the first place.

– Carl P. Leubsdorf is Washington bureau chief of the Dallas Morning News.