Ad Hominem Attack

This is also called Argumentum ad hominem or Personal Attack.

This fallacy is a deliberate distraction (Red Herring) in which a proponent of a premise is criticized rather than his argumentation. The speaker is bad therefore what he says is false.

In common parlance this is called killing the messenger.

Examples

A political candidate is associated with, but didn't write some racist newsletters from 20 years ago, therefore what he says today about the vast criminal activities of powerful bankers (unrelated topic) is false.

The whistleblower is alleged to have failed to pay a tip to a waiter ten years ago, therefore his claim that XYZ company polluted the river is false.

Hitler was reported to have said once that 1 + 1 equals 2. Therefore 1 + 1 is not 2.

Weaknesses

Ad Hominem is a simple and devious tactic.

One only needs to ask: Does it really matter who expresses an idea? The answer is obviously no. A truth is true whether it is expressed by a good person or a bad person or spoken by a computer or a parrot. Direct your attention to the claim itself.

You can draw attention to the fact that others make the same claim as the victim of the Ad Hominem made. Perhaps someone whom the attacker admires also said it. Perhaps a multitide of people did.

The attacker does not want you to question the bad things he has to say about his victim, which are usually fabrications to begin with. He may attack you for doing so.

The attacker does not want to himself be attacked using Ad Hominem, but for parity's sake you could do so.

The attacker may be guilty of far worse things than his claim about his victim, or the same things. This is often the case with those who attack (or pay others to attack) whistleblowers.

The attacker may take measures to avoid being counter-attacked: He may attack anonymously as with an anonymous editorial in a newspaper. They may attack through a proxy. She may spread wild rumors and the victim will not know from whom the rumors are originating.

The attacker will probably use an array of emotive terms, such as discredited, disgraced, and conspiracy theorist, to marginalize and denigrate the victim rather than honestly debate the victim's claim.

The attack may resort to ridicule as part of the attack on the proponent's character.

Recognize the desperation inherent in character assassination. You should ask what the attacker's desperation means about the attacker's own claims about related matters. For instance, does the company that's attacking the whistleblower believe that XYZ type of crime is OK?

The attacker may expand the character assassination, inventing victims for his victim, the more the better. Demand hard proof, because there likely won't be any.