For over a week now, Obama has held a 5 - 8 point lead in the national polls, and this lead has carried over into the individual states too (i.e. it is not that he is just wildly popular in California). By most metrics, were the election to be held today, Obama would cruise to victory, getting more than 300 electoral votes and maybe even bumping up against 350.
We go the polls in 4 weeks. The only remaining official events are the second (tomorrow) and third (Oct. 15) presidential debates. Is there any way McCain can turn this around and win? Not likely at all in my opinion. I highly doubt that Obama will make a mistake at the debates --including the primaries, he's going on about two dozen of them, and has yet to make any big gaffes yet.
I'm sure that the McCain campaign is going to turn quite negative, but does that mud really stick this late in the game, especially when you and your surrogates have been throwing mud for months anyway? October is not August, and I think it is very hard to change people's perceptions this late in the game -- the sort of emotional connection you do (or don't) have with a candidate is probably pretty well set right now. McCain essentially needs an external event to intervene, and probably nothing short of a terrorist attack would help him (that's a weird sentence to write, and I feel a little dirty for having done so).
One interesting question is how much this economic crisis hurt McCain, as it occured at essentially the same time that the American public learned about the idiocy that is Sarah Palin. I actually think that the latter event drove McCain's numbers down more than the former. People don't understand the economic crisis, but they do understand that Palin can't speak in sentences.