Why did countries with two-round systems (TRS) switch to proportional representation (PR) in early 20th century Europe? One of the major criticisms to the traditional ‘left threat’ account, which points to PR as a means for elites to contain the rise of challenger parties, like the Socialists, is that TRS already allowed bourgeois parties to lend each other votes to fend off challengers. We argue that, in these contexts, PR was not introduced to prevent a potential failure of elite coordination, but to prevent elite cooptation of challenger parties and their voters. In fact, TRS rewarded disproportionately those incumbents who were willing to coopt parts of the challenger's agenda and electoral base. This centripetal bias of TRS generates a peculiar pro-PR coalition, comprising both conservative elites unwilling to engage in cooptation and the challenger parties themselves. The theory was developed in an abductive process, shifting between two case studies and a formalised model. We assess the observable implications of the theory based on France and Italy, both countries that abandoned TRS in 1919. Our analysis of primary sources from electoral reform advocacy literature, parliamentary proceedings and party publications supports that anti-cooptational rationales for PR adoption strongly informed actors' alignment and understanding of electoral reform. We conclude by assessing the external validity of the theory.