Here's why the third debate, and all three debates, helped Obama so much more than McCain.
In general-election debates, it's a losing strategy to "rally the base." That's what your own campaign events, and your fund-raisers, and your targeted ads, and your running mate are for. Especially by the time of the second and third debates, the job is to "rally the center." That's where most of remaining persuadable and undecided voters are.
Everything about Barack Obama's approach to this debate, and all debates, was consistent with this reality. Almost nothing about John McCain's approach was:
- Obama took every opportunity to steer questions back from campaign tactics to governing issues. ("It's been a tough campaign, and we have hurt feelings, but what really matters is avoiding four more years of...." All quotes here are from memory and therefore approximate, but true to the general spirit.)
- He took every opportunity to talk about "working together" to deal with those issues, ("The reality is, it's going to take Republicans and Democrats working together.")
- He took nearly every opportunity to suggest encompassing rather than polarizing approaches to the substance of those issues. ("Do we want to reduce the cost of health care or expand the coverage? We've got to do both...")
- He took every opportunity to identify areas where he and John McCain actually agreed on approaches. ("I agree with John..." might have seemed an over-used trope in the first debate. This time, very selectively, it helped in the control-the-center strategy.)
- He took most opportunities to remain calm, to stay above the fray, to seem amused rather than frazzled, not to take personal offense. As mentioned earlier, he was not quite as perfectly self-contained as in earlier performances. But compared with McCain, he was the one -- in a good sense -- who had taken Prozac, while McCain seemed to be in a 'roid rage. And because of this general self-possession -- realizing, for instance, that there was only upside in being gracious about Sarah Palin -- when he decided to bear down, as in the breathtaking "At your running mate's rallies, when someone mentions my name they say 'Terrorist' and 'Kill him,'" it was the more powerful.
If you go down the same list, you can see that McCain did just about the opposite on every one of the counts. His most effective rhetorical line was that if Obama wanted to run against President Bush, he could have done so four years ago. (For that matter, so could McCain.) But that was undercut, according to the logic above, by emphasizing tactics over issues, by emphasizing partisan division over conciliation, by body-language contempt for his opponent, and by a demeanor that reinforced the short-tempered and dyspeptic impression from the previous debates.
Whatever the instant polls said, however you lined up the debating flow, the person who was already ahead had a plan that could gain him more support, and the one who was behind played to the base.