Is there a name for a logical fallacy where person A criticizes X, and person B fallaciously assumes that because A criticizes X he must therefore subscribe to position Y, the presumed opposition of X, although A does not, in fact, take that position? For example, if A criticizes a Republican policy then B assumes that A must be a Democrat and staunch Obama-supporter,even though A is in fact a Republican himself, or else an Undeclared who regularly criticizes Obama as well.

It seems to be a special case of a fallacy with many names: 'false dichotomy,' 'false dilemma,' 'black-and-white thinking' and 'either/or fallacy' are among the more common. When someone commits the fallacy of the false dichotomy, they overlook alternatives. Schematically, they assume that either X or Y must be true, and therefore that if X is false, Y must be true. The fallacy is in failing to notice that X and Y aren't the only alternatives. Your example makes the point. You've imagined someone assuming that either I accept a particular Republican policy X or I am a Democrat, when -- as you point out -- there are other possibilities.

The situation you describe is a little more specific: the fallacious reasoner is making an inference from what someone is prepared to criticize. As far as I know, there's no special name for this special case, but the mistake is the same: overlooking relevant alternatives.

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