From the start, the 2004 campaign has seemed destined to pivot around questions of war, with the candidates battling over the direction of US policy in Iraq and how best to defend against the threat of terrorism.
But lately, it's another conflagration - Vietnam - that's dominating the campaign discourse.
From the Bush team's raising questions about Sen. John Kerry's Purple Hearts and the ribbons or medals he threw at an antiwar protest, to Mr. Kerry's questioning whether the president actually fulfilled his duty in the National Guard, the campaigns have been issuing some of their sharpest barbs - and drawing their sharpest distinctions - over a war that took place more than 30 years ago.
The backward focus may in part reflect the fact that the candidates' positions on the current conflicts, such as Iraq, are not all that different: Although Kerry criticizes the way the president took the nation to war, he is not calling for any kind of withdrawal, while Mr. Bush, for his part, has been steadily moving toward Kerry's position of greater UN involvement.
But the campaigns are also evoking Vietnam as a key window into each candidate's character - and using it to question their opponent's overall fitness to be commander in chief.
Analysts say the ferocity of the debate reveals the extent to which the battle over national security credentials is likely to pivot on how voters feel about the candidates' backgrounds and personal qualities, rather than specific policy proposals.
"I never have thought this issue was going to be won by the Bush people kicking at Kerry on policy questions," says Democratic strategist Bill Carrick. "It's a more subjective intangible - about what kind of person they think John Kerry is."
Questions of character are particularly dominant in this early stage of the campaign, as both sides scramble to fix an image of Kerry in the eyes of voters.
The Bush campaign has sought to portray Kerry as a flip-flopper, and has linked that characteristic directly to national security with an attack advertisement highlighting a statement Kerry made about how he voted in favor of $87 billion for the troops in Iraq before voting against it.
And while the Bush team has been careful not to criticize Kerry's Vietnam service, it has increasingly highlighted Kerry's antiwar activities after he returned - and implicitly raised questions about his integrity and veracity.
Bush adviser Karen Hughes told CNN she was "troubled" that Kerry only pretended to throw his medals away at an antiwar protest - a claim that Kerry and others have directly disputed - saying, "I can understand if, out of conscience, you take a principled stand and you would decide that you were so opposed to this that you would actually throw your medals. But to pretend to do so, I think that's very revealing."
Analysts from both parties agree that the effort to define Kerry as unprincipled and waffling has taken a toll, pushing up his negative ratings in battleground states.
Moreover, while candidates such as Bill Clinton have successfully overcome a "flip-flopping" label in the past, it may prove more devastating in an election where national security is a top issue - since voters are more likely to want a leader who's strong and decisive.
"The Bush team has Kerry on three sides of every two-sided issue," says Scott Reed, a Republican strategist. "What it has done is cause Kerry to not be able to rise to the level of being a potential commander in chief."
On the other hand, Democrats point out that Kerry remains tied with Bush overall in battleground states - and that he has only just begun to define himself, through positive ads.
Polls show voters are unhappy with the current direction of the country and looking for change, which may make them especially open to hearing Kerry's story.
"They're looking for somebody who crosses the threshold of credibility on big issues that they can't get past, like national security," said Democratic pollster Stan Greenberg, in a recent conference call with reporters. "And so I think people are looking for a story that shows, based on his military experience ... that this is someone you can trust to take on this issue. I assume Kerry will do that," he adds.
Moreover, while it may be necessary for the Bush campaign to try to take some of the shine off Kerry's Vietnam service, it's a tactic Democrats believe might backfire. For one thing, the Kerry campaign has already shown a willingness to attack back - with the candidate himself challenging Bush to prove he fulfilled his obligation to the National Guard.
Indeed, the focus on what Kerry did 30 years ago led the press to reexamine what Bush was doing during that time, notes Mr. Carrick. "Real people are going to relate better to a Vietnam veteran than they are to Bush's military record or lack thereof."
And while Republicans are portraying Kerry as a waffler, Democrats have been painting Bush as someone who rushed the nation into an unnecessary war and can't relate to veterans.
"He lives in a glass house when he attacks John Kerry for his service to this country," said former Sen. Max Cleland, a Vietnam veteran and Kerry supporter, in a recent conference call. Veterans would rather have someone who's "gotten a few holes in his T-shirt" in the White House.