The New York Times/CBS poll out today (write-up here) has some useful insights about why the Palin pick may weigh McCain down.
For one thing, her upside is fairly limited. She isn't fundamentally scrambling the demographics of the race, as some worried/predicted at the time of the announcement. Non-conservative women, in particular, don't appear to be taking the bait, possibly because of her hard-line positions on social issues:
[T]he Times/CBS News poll suggested that Ms. Palin’s selection has, to date, helped Mr. McCain only among Republican base voters; there was no evidence of significantly increased support for him among women in general. White women were evenly divided between Mr. McCain and Mr. Obama; before the conventions, Mr. McCain led Mr. Obama among white women, 44 percent to 37 percent.
Generating enthusiasm among the base isn't nothing. On the other hand, the base was mostly going to come home as we approached Election Day anyway. A more moderate pick would have generated less enthusiasm coming out of the convention, but could have improved McCain's standing among independents.
Relatedly, Palin's numbers suggest that even voters who like her don't necessarily think she's qualified to be president--one reason even the favorable buzz hasn't necessarily translated into votes. (The other explanation: she's number two on the ticket.) For example, even though 40 percent of voters have a favorable impression of Palin, 75 percent believed "Mr. McCain had picked Ms. Palin more to help him win the election than because he thought that she was well qualified to be president." That means more than one-third of the people who like her think she was a cynical pick.
Finally, insofar as the Palin pick forced McCain to largely ditch his critique of Obama's readiness and try to out-change him instead, it's looking like a serious miscalculation. The Times poll casts big doubts on whether he can make that contest respectable:
In one of the sharpest differences highlighted in the poll, 37 percent said that Mr. McCain would bring real change to Washington, up from 28 percent before the two parties’ conventions. But 65 percent of those polled said that Mr. Obama would bring real change to Washington.
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.
Update: This new Pew poll says McCain has opened up a lead among independents and suggests Palin--who has a high favorable/unfavorable ratio among that group--is at least partly responsible. My hunch is that the poll is a little outdated--it was conducted from September 9-14, when McCain was still enjoying his bounce, and before the financial crisis really broke through. (As Nate says, a bounce is only as good as its resilience to the first major, post-convention development.) By contrast, the Times was in the field from September 12 -16. But you can judge for yourself...