argumentum ad hominem
An attempt to prove someone wrong by trying to discredit his person, instead of arguing against his ideas. Generally used by people who disagree with someone but cannot find anything logically wrong with their statements.
Considered a logical fallacy, except by many political candidates running campaign ads. This exception to the rules of logic is easily justified by a quote attributed to Mark Twain:
Suppose you were an idiot.
Suppose you were a member of Congress.
But I am repeating myself.
Never mind the quote itself is an argumentum ad hominem.