Social psychological models of group identity and collective action should be particularly adept at providing psychological explanations for growing rates of populism in the Western World. Because populism tends to arise in times of societal shifts that reflect both economic and cultural changes, populist attitudes are likely grounded in perceptions of intergroup relations and collective attitudes. We surveyed 95 demonstrators at the 2016 Republican National Convention and 108 demonstrators at the Democratic National Convention. Results support the idea that relative anger prototypicality, in this case, the extent to which people believe that their own anger toward politicians is representative of most Americans’ anger, predicts feelings of group-based relative deprivation. Importantly, these feelings of deprivation mediate the relationship between prototypical anger and populist attitudes. These findings provide a unique picture of current political engagement, motivated by feelings of shared anger and collective feelings of lacking representation and voice.