Laboratory experiments indicate that many people willingly contribute to public goods and punish free riders at a personal cost. We hypothesize that these individuals, called strong reciprocators, allow political parties to overcome collective action problems, thereby allowing those organizations to compete for scarce resources and to produce public goods for like-minded individuals. Using a series of laboratory games, we examine whether partisans contribute to public goods and punish free riders at a greater rate than nonpartisans. The results show that partisans are more likely than nonpartisans to contribute to public goods and to engage in costly punishment. Given the broad theoretical literature on altruistic punishment and group selection as well as our own formal evolutionary model, we hypothesize that it is being a partisan that makes an individual more likely to be a strong reciprocator and not vice versa.