To me, argumentum ad hominem is one of the weakest debating tactics possible, because everyone should readily see it for what it is. Obviously one side is using lies and personal attacks to weaken the opponent's credibility. But it surprises me how well an ad hom argument can work if it's maintained long enough and the audience is primed to believe it.
I heard a lot of ad hom attacks during the 2008 and 2012 presidential elections. If a different person had said the exact same things Barack Obama did, very few people would have a problem with it. But the ad hom argument that he was a Kenyan citizen with a Muslim agenda made his campaign pledges sound less credible.