This paper presents and tests a theory of how negative political advertising affects individuals in US elections. The theory begins with the claim that negative advertising depresses public mood. Mood is then used as information in judgments, it is argued, but more by low than by high political sophisticates: high sophisticates recognize and discount the effects of the advertisements, low sophisticates do not, instead attributing their depressed mood to their own inability to understand politics, to the political system, or to an aspect of the election such as an unappealing choice of candidates. These judgments may then affect behavior, implying that negative advertising campaigns may alter the composition of the electorate in US elections. The theory is tested in a survey experiment on a national sample. Analysis confirms the impact of negative advertising on public mood, and some relationship from mood to judgments for low sophisticates.