It's a truism in politics that all races tighten, but the reasons why are not particularly well understood. Here's one possible reason - check out the "enthusiasm gap" portion of today's WaPo-ABC Tracker release:
Obama continues to benefit from an "enthusiasm gap," but McCain has narrowed the divide this week. 64 percent of Obama's backers are "very enthusiastic" about his candidacy; among McCain's, it's 40 percent (up from 31 percent before the final debate).
The release is obviously implying that there is some causative* link between the final debate and the change in the enthusiasm numbers, but I imagine that in any race with a big enthusiasm gap, the candidate with less enthusiasm probably enjoys a big bump in the final weeks of the campaign. Obviously you can be enthusiastic about canvassing, making phone calls, organizing, or any of the other fabulous things people do early in campaigns.
But if you're not one of the people who does these things (and most people don't) there is usually little reason to feel enthusiastic about a presidential candidate in September. Meanwhile, there's probably some hard limit to the number of people you can get to feel enthusiastic about a presidential election. Some people aren't turned on by politics in that way.
I would imagine Obama is close to his natural limit already - there's obviously been an unusual level of buzz about him for some time. Meanwhile a lot of McCain supporters are still feeling sluggish, but as the election approaches you expect more and more of them to move into the "excited" column as long as someone, somewhere is telling them that McCain has a shot to win.
Currently if you look at the poll weighting data (even that of Republican-leaning pollsters like Rasmussen,) it is expected that a large plurality of the voters who would show up to an election if it were held today are Democrats. Given Obama's giant lead on most questions dealing with both substantive issues and "character and values" questions, that means Obama will win - the primary reason most McCain voters are voting for McCain is because he's a Republican, and that reason doesn't hold a lot of sway with independent voters.
What McCain needs to tighten the race is for Republicans who don't currently plan to show up to vote to get excited about the race in the last two weeks. It's likely to happen. . . and if the trend on the enthusiasm question is real and not just statistical noise, it's already happening.
Stay vigilant - it ain't over.