Ну, в основном, это - шутка, навеянная лозунгом "You voted to Obama to prove that you are not racist, now vote against him to prove that you are not stupid".
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to whom exactly would these voters be proving that they are in fact not racist, and considering the privacy of the ballot booth, how exactly would they go about documenting this proof, and why? using your logic, I guess all those who voted for GW must have done so to prove that they do not discriminate against imbeciles, and because GW isn't the sharpest tool in the shed, they had to do it twice.