Think again, according to a careful meta-analysis of the literature:
To state the matter bluntly: There is no consistent evidence in the research literature that negative political campaigning “works” in achieving the electoral results that attackers desire. Although attacks probably do undermine evaluations of the candidates they target, they usually bring evaluations of the attackers down even more, and the net effect on vote choice is nil.
That's a helluva finding, but I doubt you'll see a sudden end to negative ads. We're addicted to them. They energize the base, they provide conversation fodder. They give the talking heads on the boob tube something to talk about.
The results on political knowledge are more consistent, with 11 of 15 studies showing that negative ads increase what people know about a campaign. I'm not sure that's good news, but it is something. This reminds me of the knowledge gap literature, which in part finds that controversy spurs learning. Negative ads get attention, which increases knowledge.
But, according to this study, they simply don't persuade.