The debate season is nearly upon us - it starts this Friday evening, and ends on Oct. 15 - and it's not hyperbole to suggest that the outcome of this presidential election could hinge on a few defining moments or lingering impressions. Substance takes second priority, as we have generally seen before.

Not even the most dedicated political junkie can quote anything that was said during the first TV-era debates, between Kennedy and Nixon, in 1960; what mattered that autumn was Kennedy's telegenic poise, in contrast to Nixon's pallor (he had been sick, and he refused the offer of decent studio makeup). The '76 debates turned on President Ford's verbal blunder about Poland. The '80 debates turned on Ronald Reagan's "there you go, again." The '84 debates turned on Reagan's quip about his opponent's comparative youth. The '92 debates turned on the senior President Bush's impatient glance at his watch. The '96 debates between Bill Clinton and Bob Dole were a snooze, and I know, because I was there. The '00 debates were about Al Gore's condescending sighs, which fatally turned off a lot of women voters.

As Barack Obama and John McCain prepare to square off three nights from now, it's important to keep in mind the prospective voters who could actually be most influenced. I'm not talking about the Democratic and Republican partisans who likely will constitute 90 percent of the viewing audience; their minds are already fixed and unchangeable. If Obama came on stage and quoted from the collected works of Jeremiah Wright, his fans would probably stick with him. If McCain brought the Beach Boys on stage to help him sing "Bomb, Bomb Iran," his fans would probably stick with him. No, I'm talking here about the undecided swing voters - many of whom are only beginning to tune into this race, many of whom are not particularly versed in the nuances of the issues, and who therefore may well be guided by their gut.

They'll want to get a feel for whether this new Obama guy is presidential, and whether he exudes both toughness and empathy - or whether he seems too untested or aloof or "exotic." They'll want to get a feel for whether McCain is in touch with their everyday lives, and whether he seems like a different kind of Republican (as opposed to the Bush type) - or whether he seems too old or cranky or bellicose.

It would appear, at first glance anyway, that McCain has the upper hand in the first debate, simply because the main theme of the event - foreign policy - plays to his perceived strength. After all, the latest NBC-Wall Street Journal poll reports that 51 percent of Americans view McCain as the better prospective "commander-in-chief," while only 29 percent choose Obama. But the thing is, the Obama people insisted months ago that the first debate should focus primarily on foreign policy.

This insistence might seem like a mistake - especially now, with the economy (a traditional Democratic issue advantage) looming so large, with McCain flailing around (a deregulator one day, a fervent regulator another day), and indeed with Obama getting a lift in the latest batch of polls precisely because of the heightened economic anxiety (a new Pew Research Center survey, out today, reports that Obama is favored by a 12-point margin - and, among independents, by a 14-point margin - as the candidate who would best handle the current the financial crisis). But the Obama people apparently wanted to take a page from the Karl Rove playbook and take a run directly at the opponent's perceived strong suit, with the intention of neutralizing it or, even better, turning it into a weakness. The thinking is that if Obama can at least duel McCain to a draw on foreign policy, Obama will have enhanced his commander creds and positioned him all the better to trump McCain on the domestic side in debates two and three.

Assuming, for the moment, that issues do matter, Obama on Friday night can be expected to build on some of the lines he delivered in his nomination acceptance speech - about how, in his view, McCain is a George W. Bush clone who has screwed up the war on terror, and potentially made America weaker, by cheerleading from the outset for the money-draining, military-straining debacle in Iraq. Obama needs to paint McCain as a bellicose and reckless guy with no foreign-policy vision (it would be a bonus if he can get McCain ticked off, thus leaving the impression that McCain lacks the requisite temperament for the job), and he needs to make this stick - because McCain will surely try to paint him as an inexperienced Bambi who doesn't know the dangers of the forest. Indeed, McCain will bang Obama for not supporting the Iraq "surge," for refusing to admit that his initial pessimism was wrong - and McCain will try to frame this episode as proof that Obama is insufficiently seasoned to lead. McCain's basic take on foreign policy is visceral, and hence ideal for TV: we gotta win, bad guys gotta lose. Obama's basic take - diplomacy, other incentives, military as last resort - is less suited for instant home consumption; he'll need to do his nuance while avoiding McCain's efforts to tag him with the Democratic wimp stereotype.

But, as I signaled earlier, the way they communicate their points is arguably more important than the points themselves. Obama - who I don't believe was particularly impressive in many of the Democratic primary season debates - will need to tame his penchant for ruminating at circuitous length before he gets around to addressing a question. He needs to be a boxer, not a law professor. He needs to deliver short punchy responses that sting his opponent. McCain's challenge is almost the reverse. He already speaks bluntly and declaratively, so he'll need to avoid leaving the impression that he sees the world as simplistically black and white. He'd probably also do well to avoid delivering his attacks with the frozen sneer-grimace that is supposed to pass for a smile, but instead makes it appear that he is undergoing a rectal exam.

One other factor: Even though the topic is foreign policy, the domestic economic woes will surely come into play. Both candidates, presumably, will find an opportunity to feel the pain of the viewers at home; somebody will find a way to argue that America's strength abroad begins with being strong at home, and take it from there. All told, the Friday night debate is ultimately about leaving a good first impression with the prospective voters who are new to the contest; it's all about projecting empathy, toughness, judgment, temperament, and the kind of indefinable aura that would make the person welcome on the kitchen counter TV for the next eight years.

And that's how it was when the debate era dawned in 1960. Which is why, by election day, virtually nobody remembered a single word of the lengthy, substantive debate between Kennedy and Nixon over the future status of Quemoy and Matsu.

Click here for Philly.com's politics page.