I might be the only person in the blogosphere (or at least, one of the few) who defends negative advertising, and a recent survey by YouGov shows that voters aren’t necessarily turned off by negative ads - depending on their content. As the charts from YouGov show, voters find negative ads which point out differences between the candidates to be helpful; however, ads which criticize a candidate’s family to not be helpful (there are some other charts as well). As YouGov notes, positive ads usually only offer platitudes and little substance, whereas negative ads - more so if they’re done correctly - offer more information for voters about the candidates (because they’re telling you things about the candidate he/her wouldn’t say about themselves).
I’m not ashamed to be a fan of negative ads - with one caveat: they have to be factual. A negative that isn’t factual will not be effective and will backfire on the candidate.