Joe Klein thinks negative ads have been more effective this cycle because no one has to claim ownership:
It became harder to for a candidate to have an ad accusing an opponent of being a mother-raper if he or she had to appear at the end and say, "I approve this message." In fact, in 2004, "I approve this message" just about killed Dick Gephardt in Iowa, as he set to work filleting Howard Dean. Iowans are nice. They don’t like candidates who aren’t.
This time, however, the vast majority of Iowans don’t know that friends of Mitt Romney have put several bajillion dollars worth of ads up eviscerating Newt Gingrich.
The Dish 