The literature on voting behavior has generally accepted that party identification largely determines voter choice. While many studies have found that party identification is largely transmitted through social learning, less studied are the processes of the construction of party identity by way of group membership. This study seeks to understand how group identity influences party identification among Mexican workers through an analysis of the effects of union affiliation on political behavior. It assesses the utility of corporatist legacies in explaining party identity in Mexico and provides a first assessment of party affinities among independent unionists. The evidence draws from original survey data collected during six demonstrations in Mexico City. The study finds that union membership does condition the party identity of corporatist workers but not that of independent unionists.