I think the choice will backfire in any combination of three ways.

a) Palin will end up saying something so mind blowingly stupid that even her closest allies hang their heads in shame.
b) People will forget that McCain is even running for President thus diminishing his vote even further.
c) Intelligent conservatives will grit their teeth and vote for Obama, simply because he is ultimately a more responsible candidate.

I just had the joy of watching Warren Beatty's "Bulworth". Now there's an American politician I could like!