Political debate is fertile ground for this fallacy. Heck, just about any arena of debate is a perfect breeding ground.
Again, the following analysis is not meant to the definitive work on the subject.
Defined
The ad hominem is the classic fallacy of attack the messenger and not the message. When someone employs this fallacy, it is used without evidence about the person attacked, and most importantly, it does not offer evidence to refute the argument being made. This type of attack works like a straw man. It diverts attention from the real issue.
The ad homimen can take the form of using a label, like conservative or liberal, as a pejorative. A sure sign of this fallacy is abusive language, name calling, or unwarranted anger against one's opponent.
Also, the ad homimen may take the form of Poisoning the Well, in order to breed suspicion and fear around a person. Additionally, I've written about a special plead with patriotism used as an ad homimen.
In many cases, the petitioner will use an ad hominem with the full knowledge that they are doing so.
These examples were culled at LCL.com.
Examples
A. "Nothing is bizarre about the fact that liberals are as bad as terrorists. Both wish to destroy the United States." (Amy L. Geiger-Hemmer)
B. "That is another hypocritical, liberal lying statement from you." (Jim Hayett)
And a bit of sarcasm aping the first example:
C. "McCain evil, he eats small children and watches Gossip Girls....McCain bad man....ooh icky." (Victor Ponelis)
Example A: Uses abusive language by conflating two unlike groups of people in the form of a false comparison.
Example B: Not only is the language abusive, but it makes two accusations without evidence and turns a type person (like a liberal) into a pejorative. A person may be a liar or hypocrite, however, that does not automatically disqualify the argument by that person.
Example C: Effectively makes fun of A and is funny! (None of it true of course - well except for maybe the tv show - but I like Gossip Girl.) This tease accomplishes in a few words what I've written here.
In Conclusion
Arguments should be honestly confronted and rejoined on the merits of the position, not from the anger of the petitioner toward the person offering the argument.
When a petitioner sinks to the level of an ad hominem, we can safely conclude that the petitioner either does not have a valid argument or has already lost the debate. In the words of one Lake Country poster, jmark, it is "middle school name calling."