Maybe Romney will pull it off.  But more often than not, what determines the outcome of debates is not the zingers, but the gaffes the candidates make — as when President Ford said that Eastern Europe was not under Soviet domination, or when President Carter said that he had discussed nuclear weapons policy with his 12-year-old daughter, Amy. Goal one for both candidates is to avoid being the one who emerges with that “Amy Carter moment.”

This is a reality many pundits seem to miss. Back in 2004, the media was almost universal in their judgment that John Kerry had won his three debates with George W, Bush — and by Cambridge Union rules, they were probably right. 

But the truth is, Bush won — because he emerged from those debates gaffe-free, while Kerry had two major gaffes that had lasting effect on his campaign:  First, he commented on the sexual persuasion of Vice President Cheney’s daughter, Mary Cheney, and second, he said that the United States had to pass a “global test” before committing U.S. forces to combat. 

Those gaffes were what Americans were talking about after everything was all over. As the New York Times reported after the debates:  

Mr. Kerry's advisers acknowledged Sunday that some voters perceived Mr. Kerry's remark as an invasion of Ms. Cheney's privacy, a gratuitous personal insult, or a crass political calculation by which Mr. Kerry was trying to drive a wedge between Mr. Cheney and conservatives unaware that his daughter was gay. Republicans were quick to seize on the exchange to reinforce their effort to portray Mr. Kerry in these closing days of the presidential race as a man who, as Mr. Cheney put it, "will say and do anything in order to get elected."  "He shouldn't have done it," said Matthew Dowd, a senior adviser to Mr. Bush. "It was inappropriate. I just don't think you should bring up people's children in the course of a campaign. And it wasn't just accidental that he did it — he's not an accidental guy."

As for the “global test” comment, it became a staple of Bush’s campaign stump speech (“When our country's in danger, the president's job is not to take an international poll. The president's job is to defend America”) and fodder for a devastating campaign ad:

So while the commentators all said Bush had lost, the fact is, Bush came out of the debates with new ammunition for campaign ads, while giving none to his opponent.  That’s a win.

Here’s a metric for how you can tell who won tonight: Does either side cut an ad tomorrow quoting what the other candidate said in the debate?  Whoever is being quoted in his opponent’s next ad lost.