purchasing elections

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In Mexico, there is a fear that the up-coming elections will be corrupted by narco-money -- that drug cartels will buy the political process that is theoretically in the hands of the electorate. There are those who claim this would be a travesty and a perversion and an eek.

Will someone explain to me how the situation in Mexico differs significantly from the purchase of the American political process by the super political action committees that were let loose by the U.S. Supreme Court's "Citizens United" decision?
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