I know, it was painful to listen to. We know Romney's lying; we know Obama knows he's lying (or if we want to be kind, showing a massive case of entitlement to his own facts). And we wanted to hear Obama nail him to the podium with it. But Obama was probably right not to do it.
Here's the problem: when Romney, running a campaign based on ignoring the facts, accuses Obama of being the one who's misleading voters, there are two serious dangers to making the intuitive, immediate response, the response we all wanted to hear. They're dangers of style rather than substance, but style matters: it may be the thing that matters most in these events. The first is that anger plays badly from a president, and for historical reasons it would play particularly badly from this one. (Even the whitest of presidents can only really get away with being visibly angry in response to a personal attack on a family member or a physical attack on the nation.) And with Romney already struggling with the impression that his lurches into You People and How Dare You leaves on voters, the last thing we should want is for Obama to make that impression less clear by doing a How Dare You himself on national television -- which is what "You are calling me a liar!?" risks becoming by its very nature.
The second danger, subtle but important, is that of Obama diminishing his own stature in the public's eyes. The first person to call liar in a serious, adult event might be able to get away with it, though there's always a risk of it backfiring. But being the person who says, No, you're the liar? No matter how true it may be, it's almost guaranteed to make that person look childish and tiny. Again, not what you want to do in a presidential debate on national television. In fact, I suspect that was what Romney and his people were hoping for.
No, you kind of have to do what Obama did. Which is, stay calm and put out a massive memo to the news media the instant it's over, highlighting all the real howlers, and wait for them to do their jobs. It's unfortunate, but I really don't see what else he could have done. And by staying calm and letting Romney walk out onto factual ice so thin we could all hear it cracking under him, in front of all America, Team Obama may have actually put Romney into a much worse position than when the debate began.
And it's too early to call this a debate loss. Some here may remember that Al Gore won his first debate with Bush back in 2000. For about twelve hours, before the media really got going. After that, not so much. The narrative is almost guaranteed to be more splintered this time out, but if even a quarter of the mainstream press goes with "Romney sounds smooth, repeats many previously-debunked claims," things are going to look very different by the weekend.