I think McCain's only hope is to shift the topic back to national security--or to pray for some external development (or some Bush administration monkey-business) that does it for him. According to the Times, "Forty-eight percent said Mr. Obama was prepared enough to be president, compared with 71 percent who rated Mr. McCain as adequately prepared."
But, of course, it would have been much easier to press that advantage if his running mate weren't in her second year as governor of Alaska.
Palin is a swing and a miss to this campaign precisely because McCain can't go back to his national security policies with any credibility anymore. And while experience may be balanced out between the tickets, as Jonathan Chait points out:
Engagement, not experience, is the difference between Palin's qualifications and Obama's. Obama has a longstanding interest in national and (to a lesser extent) international issues, and has answered questions on all those issues in extensive detail. Palin has dealt almost exclusively with parochial issues in a wildly atypical state.
Essentially, Palin's popularit is crumbling around her. Independents are starting to realize that much of what drew them to her in the first place--her charisma, what appeared to be a no-nonsense reform agenda--is all a lot of smoke and mirrors.