Olga, yes. Too deep in sugar coma to think of anything clever, but here is mine:

Obama- as a valid presidential candidate , easy as he already has the costume
Biden - as a man that has an original thought. in his head .. a really difficult costume for him, but perhaps he can plagiarize from someone
McCain as Archie Bunker, no good reason, just think that would be funny and piss off a whole bunch of lefties
Palin as a naughty nun, no good reason, just think that would be funny and piss off a whole bunch of religious freaks
;-)

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you can't always eat lobster, sometimes you gotta have fish (so says my son)