Given the depth of research on negative advertising in campaigns, scholars have wondered why candidates continue to attack their opponents. We build on this research by considering real-world campaign contexts in which candidates are working in competition with each other and have to react to the decisions of the opposing campaign. Our results suggest that it is never efficacious for candidates to run attack ads, but running positive ads can increase a candidate’s margin of victory. These results are conditioned by two factors: candidates must both stay positive and out-advertise their opponent. Second, the effects of positive advertising are strongest in areas where the candidate is losing or winning by a large margin—areas where they might be tempted to not advertise at all.