The latin "ad hominem", translated literally, means, "to the man". It is used in two ways: to appeal to one's prejudices, emotions, or special interests rather than to one's intellect or reason, and, as we've seen ad nauseum (yeah, yeah) in the last two years, attacking an opponent's character rather than answering his argument.
Generally, people in positions of authority don't feel it necessary to engage in ad hominem, it being perceived as unbecoming and an abuse of that authority. So when somebody in authority - say, a President - does personally attack somebody solely for that person's expressed opinions, we have to wonder why. When President Obama told Congressional Republicans last week that, "You can't just listen to Rush Limbaugh and get things done," he was beneath the office of President of the United States. Now, Rush certainly doesn't need for me to come to his defense: he's possessed of intellect and skills of erudition that will run rings around both me and Mr. Obama. My point is that the President should be justifying his initiatives on their own merit, not by taking swipes at a private citizen - one of the citizens to whom he is accountable. Methinks he took Saul Alinsky's advice too much to heart: to paraphrase Mr. Alinsky, "Destroy everybody who doesn't agree with you." Is this the conduct and mindset we need in a chief executive?