An ad hominem attack is a tactic whereby you attack the man instead of the validity of his argument or position. The fallacy of such an attack is that it completely ignores what is really important… Is the argument logical and accurate.  
Character assassination is an old tactic used to win battles whether they are at the ballot box or in the courtroom. If you don’t like someone, then you are less likely to side with them and more likely to forget what the case is really about and decide it on an emotional basis. In the Karen Silkwood case, Gerry Spence used the following analogy to point this out to the jury and counteract the defense’s tactics of misdirection and obfuscation leveled against Karen Silkwood who gave her life courageously exposing problems at a nuclear power plant: 
“I’ll tell you a bigger defense than that — and that’s getting drowned in mud springs. My favorite jurist has an old saying. He says if you want to clear up the water, you’ve got to get the hogs out of the spring. If you can’t get the hogs out of the spring, I guarantee you can’t clean up the water. During the course of this trial, you have observed a process by which you have been invited over and over again to get down into the mud springs where you can’t see — where you can’t understand — where things are all muddy. Getting jurors confused is not a proper part of jurisprudence — and getting people down in the mud springs is not the way to try a case.”
Don’t let your opponent drag you or client through the mud. Use this analogy to point this tactic out.