For the record and from the first link in post 96
Argumentum ad hominem (argument directed at the person). This is the error of attacking the character or motives of a person who has stated an idea, rather than the idea itself. The most obvious example of this fallacy is when one debater maligns the character of another debater (e.g, "The members of the opposition are a couple of fascists!"), but this is actually not that common. A more typical manifestation of argumentum ad hominem is attacking a source of information -- for example, responding to a quotation from Richard Nixon on the subject of free trade with China by saying, "We all know Nixon was a liar and a cheat, so why should we believe anything he says?" Argumentum ad hominem also occurs when someone's arguments are discounted merely because they stand to benefit from the policy they advocate -- such as Bill Gates arguing against antitrust, rich people arguing for lower taxes, white people arguing against affirmative action, minorities arguing for affirmative action, etc. In all of these cases, the relevant question is not who makes the argument, but whether the argument is valid.
It is always bad form to use the fallacy of argumentum ad hominem. But there are some cases when it is not really a fallacy, such as when one needs to evaluate the truth of factual statements (as opposed to lines of argument or statements of value) made by interested parties. If someone has an incentive to lie about something, then it would be naive to accept his statements about that subject without question. It is also possible to restate many ad hominem arguments so as to redirect them toward ideas rather than people, such as by replacing "My opponents are fascists" with "My opponents' arguments are fascist."