I just thought of this while in the shower (where the best ideas come from!) so thought I'd share.
One of the reasons McCain was able to keep the race close for a long time was, essentially, because of Democratic defectors. Obama struggled for a long time to get to even 80% support among Democrats, while McCain enjoyed nearly 90% of support from Republicans.
Well, based on most of the polling that we've seen now, that's done. McCain and Obama draw (roughly) equal support from their parties.
What does this mean? Well, it means what we all know already... Democrats have an inherent starting advantage. But it means more than that...
If we use Rasmussen's (somewhat conservative) numbers for the electorate, we have roughly 39% D, 33% R, and 28% I. Assuming equal support from their parties, Obama starts with a 6 point lead.
So, McCain needs to make this deficit up with Independents. But how much does he need to win Independents by in order to tie Obama? The math is pretty simple:
28% * x = 6, x = ~21
So, assuming equal support from their parties and a 6 point Democratic advantage, in order to TIE Obama McCain needs to win Independents by about 21 points.
Is this possible? Well, right now, McCain is losing independents... by a lot. According to the NYTimes Poll, Obama is winning Indies by a 51-33% margin. That would mean that McCain needs a 39 point swing among independents to bring things back to a tie again, or a combination of that along with driving a wedge among Democrats again. I'm not sure how likely that is any more.
And this is (one reason) why McCain has around a
5% 4% chance to win according to Nate Silver at 538.com.