While America’s Iraq war will soon eclipse the length of World War II, it is becoming increasingly apparent that the issue that dominated the 2004 and 2006 elections may well also be the one on which the 2008 presidential campaign turns.

Depending on whether, and how many, U.S. troops are still dying there, and what the political situation is on the ground, the war could turn out to be the bane of John McCain’s existence. Or, for that matter, it could continue the Democrats’ post-Vietnam credibility problem on national security in the eyes of the American people.

McCain, many polls and insiders agree, is the leading candidate to become president in 2008. But the Arizona Republican senator’s chances to win the Oval Office are more tied to the war in Iraq than he might like.

By the same token, even though their general opposition to President George W. Bush’s Iraq policy gave Democrats a boost in 2006, the lesson of Vietnam should give them pause about how strongly they want to use their congressional majority to restrain his war policy.

The irony is that McCain, whose crossover appeal is largely based on not always toeing the GOP line, has become Bush’s biggest supporter on the war.

In fact, McCain, a career military officer who spent years as a prisoner of war in North Vietnam, has been more outspoken than the president about the need to increase, not reduce, troop levels in Iraq.

Even as many who were early and enthusiastic backers of Bush and the war have begun to trim their sails, McCain has become more outspoken about the war. With Bush now postponing until after New Year’s a decision on how to proceed, the McCain view is reportedly getting increasing attention within the White House.

If Bush does follow McCain’s advice, it’s likely that the president’s popularity will fall even more, at least in the short term. But Bush will never face the voters again.

For McCain, however, the payoff will be whether the war is still an issue in 2008 when the presidential primaries begin.

It is difficult to imagine a serious challenger for the GOP nomination taking on McCain, and by extension Bush, on the war.

However, given the opinion among the activists who dominate the Democratic nomination process, no candidate could win that party’s White House derby without being a strong opponent of the Bush war policy.

Yet the challenge for Democrats is to shed the reputation they picked up following Vietnam of being reflexively wary of using U.S. military strength to further American interests. They were at the forefront of efforts to end that war. But in the years following the U.S. withdrawal, South Vietnam, the American ally, was overrun by the communist north.

The result was a period of American angst and recrimination. U.S. voters, polls showed and subsequent presidential elections proved, decided they did not trust Democrats with the national defense.

Republicans have dominated the presidency since then, in large part because polling data showed for a quarter century that many Americans question Democrats’ toughness on national security.

Witness the effort by some Democrats with 2008 White House ambitions, especially Sen. Hillary Clinton, to walk a fine line on the Iraq question.

She voted to give Bush the power to invade Iraq but has been increasingly critical of his policy in an attempt to appeal to the Democrat’s anti-war wing, which carries great sway in the nomination process. Most other major potential Democratic aspirants have been even more outspoken against the war than she.

It is hard to imagine that more than five and a half years after the Iraq war began – which would be Election Day 2008 – it would still be a determining issue in picking a president. But it could well be.