Guilt By Association is an attempt to discredit the opponent by suggesting that because they have something in common with something discreditable, that they should likewise be dismissed.
The term 'ad hominem' is Latin for 'about the person'. This class of fallacy refers to comments about the opponent that are intended to either convince the opponent to stop arguing, or to convince the audience to stop listening to his arguments.
"So, you believe in low unemployment? That's exactly what Hitler belived!"