See this press release on research about negative ads and how, according to Goldstein in his book, they actually help democracy by informing the public.
I kinda agree, in part because advertising shows up in entertainment fare, reaching an audience largely tuned out of news, and the negative nature is more memorable than soft fuzzy friendly ads.
Some scholars argue that advertising may be the only way to reach large segments of a disconnected, indifferent public. A negative ad on such quality programming as Dancing with the Stars, for example, may go a long way.
Of course this raises the question of whether anyone who watches Dancing with the Stars (or American Idol) should be allowed to vote.