Barack Obama made a major gaffe yesterday in an interview with ABC's Terry Moran, saying that despite the obvious success of the surge he still would not support it if he were able to cast his vote again. Why? Obama told Moran that he wanted to change "the political debate."

Here is the exchange:

Obama: No, because -- keep in mind that, that … ABC: You wouldn't? Obama: Well, no, keep in mind -- these kinds of hypotheticals are very difficult. You know, hindsight is 20/20. But I think that what I am absolutely convinced of is that at that time, we had to change the political debate, because the view of the Bush administration at that time was one that I just disagreed with.

It's a moment that will provide a real test for the mainstream media covering his campaign. He should have to answer some version of these obvious follow-ups:

Would the U.S. be in better strategic position today if there had been no surge and if troops had pulled out in early 2007 as Obama had argued?

And what about the U.S. troops who would have fallen in the absence of the surge?

Isn't the fact that Prime Minister Maliki can now talk openly about a timeline for withdrawal itself evidence of the surge's importance?

Which candidate wins the debate over Iraq has everything to do with how that debate is framed. Obama wants the debate to be one of timing, McCain wants to talk about outcomes. Obama: Do we leave soon or stay indefinitely? McCain: do we fight to win or choose to lose?

McCain's team is hammering Obama on the gaffe. McCain should talk about nothing else today. The question is whether even a mistake of this significance will be enough to break Obama's good-luck streak or loosen the loving embrace of the mainstream media. There's reason to be skeptical.