Finally, we are about to reach the last of those seemingly fateful moments that seemed would never arrive in this seemingly endless campaign. Do you remember, say, mid-2007, when it was hard to imagine that the Iowa caucuses would ever actually take place? They felt then a lifetime away. But just like Christmas in Whoville after the Grinch had stolen the presents, somehow or other, they came just the same.

So the debates are upon us. The first comes this Friday, in Oxford, Mississippi, and is focused on foreign policy. By custom there won't be as much campaigning as usual this week as the candidates hunker down and study their briefing books and rehearse their zingers. And rehearse they do, obsessively. They even have supporters "play" their opponent in mock-debate settings.

And why shouldn't they? These really are important moments. They're the only three chances the people have to size the candidates up, one on one. If TV viewership of the conventions is any indication, more people will watch these debates than watched spring's American Idol finale. More is at stake in these 90-minute sessions than in any other event on the American political calendar.

You may be familiar with some of the dramatic, history-turning moments. Gerry Ford saying in 1976 that there was no Soviet domination of Poland. Ronald Reagan asking voters in 1980 to devastating effect: "Are you better off than you were four years ago?" Bill Clinton locking eyes with a citizen in 1992 who asked him to put a human face on the deficit problem, as George HW Bush stood by mute. Al Gore sighing too much in 2000.

If that last example doesn't seem of a piece with the others, there's a reason. These days, debates aren't 90 minutes. They're 72 hours. In today's American media culture of cable television and ideologically competing blogs, there is first the debate itself and then the debate over the debate. The latter is hashed out over the three days following the debate, during which the instant, debate-night conventional wisdom can be turned on its head by the side that has the more aggressive spin operation.

That spin starts during the debate itself. Both sides have teams watching the other guy and recording his misstatements and falsehoods. Emails are blasted to political journalists. By the end of the 90 minutes, the reporters may have 30 or 40 emails in their inboxes.

Then cable TV takes over. There's an instantly declared winner, based on the pundits' hunches and focus groups assembled by the networks to watch and react. On the night of the debate, everyone may generally agree that Candidate A won. And that's when the really expert spin begins. Over the next three days, the side that was declared the instant loser starts spinning that the initial spin was wrong and they actually won. Sometimes, it works.

Hence Gore in 2000. It was generally agreed the night of the first Gore-Bush debate that Gore got the better of things. But in the succeeding days, the cable nets, led by the all but openly-pro-Republican Fox, started focusing on the question of Gore's "condescending" sighs (when Bush lied about Gore's tax plan, for instance). The sighs, shown repeatedly on cable, became the story, symbolic of the "fact" that Gore was too much a smarty-pants to be president. That weekend Saturday Night Live spoofed them to great effect. The debate that Gore had won in real time was, within days, a debate he'd lost.

Candidates need not only to master the issues. They need to master performance. Cable television will replay clips; the candidate who wins the battle of the clips will be the 72-hour winner. That means getting off the better one-liners. And crucially, it means being ready with a witty riposte to the other guy's one-liners. Both sides undoubtedly have people anticipating the other fellow's zingers and crafting counter-zingers.

The topic, foreign policy, is McCain's alleged strong suit on paper - certainly he's relieved the topic isn't economic policy, especially given his confused responses to the Wall Street crisis. He will push his support of the surge in Iraq and press Obama on the latter's support for diplomacy with Iran and on Israel policy. Obama will emphasise America's low standing in the world and the need to get out of Iraq as quickly as is prudently possible. The substance doesn't matter as much as the theatre. McCain will, for instance, surely needle Obama on the experience question. Obama had better have a sharp and snappy reply to that. If he doesn't, he could lose the 72-hour debate in that single exchange. This isn't the democracy Thomas Jefferson had in mind. But as Donald Rumsfeld might say, it's the one we have.

· Michael Tomasky is editor of Guardian America

michael.tomasky@theguardian.com