This was Obama’s worst performance overall but I thought one of his strongest moments was when the discussion, at long last, came to a “values” issue — abortion.The key point to remember is that in a debate like this, at this late date in a campaign, the political goal is to position oneself as sensible and mainstream. McCain scored points that would appeal to pro-life activists, while Obama kept relentlessly focused on the ambivalent middle. McCain went after Obama; Obama went after undecided voters.The discussion began with McCain on the offense, painting Obama as the extremist. McCain focused on the two issues in which Obama seems most out of the mainstream — the “born alive” bill, involving babies born during abortions and late term abortions. Obama did an adequate job defending himself but still let McCain define the terms of the debate at that point.McCain meanwhile headed toward the middle by emphasizing not that he opposes abortion but rather that he wants the states to decide the issue. No extremism there. But for the rest of the abortion discussion, Obama outflanked McCain. It was Obama who made the call for finding common ground by reducing the number of unintended pregnancies and sent the dial-meters soaring by saying children should be taught that “sexuality is sacred.” That one phrase probably did more to cast Obama as a cultural moderate than anything he’s said in all the debates. McCain used language that would thrill pro-life activists but either alienate or confuse most centrist voters. For instance, when Obama said he would support a ban on late term abortion if it includes an exception for the health of the mother, McCain gleefully pounced on what he – and pro life activists — view as a slippery dodge by Obama. Aha! That’s the sneaky way that pro-choice activists gut a partial birth ban, activists were cheering at home. But to a typical American, not steeped in the linguistic battles of the abortion wars, it was Obama who sounded reasonable and McCain who seemed uncaring and extreme.McCain referred to those who are pro-choice as being “pro-abortion” — a turn of phrase commonly used by pro-life activists — but which may grate on the large chunk of Americans who view themselves as pro choice but not at all pro-abortion. Obama tapped that sentiment by talking about it as a difficult moral issue and saying “nobody’s pro-abortion. I think it’s always a tragic situation.”And on Roe v. Wade, Obama finally used phrasing that seemed straightforward and clear. For most of the campaign, he’s talked about “overturning Roe” and “protecting Roe” as if he were still lecturing at law school. This time he said it in clear terms: he believes that the Constitution has a right to privacy.McCain, meanwhile, gave an answer on judicial appointments that was confusing or even duplicitous. He started off clearly stating that he would impose no litmus test on abortion, selecting judges only on the basis of experience and judgment. But at the very end of the riff he said that, of course, a judge who supported Roe would be demonstrating a disqualifying amount of bad judgment. In other words, he does have a pro-life litmus test. I don’t think most people will pick up on the contradiction the way an abortion-obsessive like myself did but I do think the passage sounded confusing.What’s the importance of this one abortion exchange? The hardcore pro-life or pro-choice voters have long since decided who they’re voting for (and this debate will probably energize activists on both sides to work a bit harder for their guy). But the real prize is the undecided voter, who by definition tends to be in the middle. They’ll likely go for the man who grabs the center. They’re not culture warriors.Specifically, I think this evening will help Obama close the deal with some centrist Catholics who are leaning toward Obama because of the economy but wary about his position on abortion. They didn’t need to be convinced Obama is pro-life; they just needed to be convinced he wasn’t an extremist. If he seems moderate and reasonable then they’ll feel like they have permission to vote for him. All in all, McCain won the battle to most thrill the activists but Obama got the bigger prize: he defined himself as a reasonable seeker of the common ground..SPECIAL: WOULDA, COULDA ADDENDUM: For pro-lifers, McCain’s big lost opportunity was to point out that Obama’s support of the Freedom of Choice Act would wipe out many centrist abortion restriction laws, undermining Obama’s “abortion reduction” efforts. For pro-choicers, Obama’s big lost opportunity was failing to paint McCain as an abortion “extremist” by tethering him to the Republican platform, which calls for a ban on all abortion, including in cases of rape and incest.Plus: The transcript of the abortion portion of the debate.