How deep are the economic roots of political institutions? While some political economists have theorized that there is a natural affinity between free-trade interests and proportional electoral systems, recent research on the origins of Proportional Representation (PR) in Europe has paid little attention to the role of international trade. We bring trade theory back to the debate on the choice of electoral systems, and argue that a trade perspective provides an insightful complement to accounts based on domestic politics. To test implications of the theory, we leverage historical data from referendums on trade policy and the introduction of PR at the district level in Switzerland 1880-1918. This setting enables us to rule out alternative explanations using state fixed effects, district-level covariates, and an instrumental variable approach. In the decade before World War I, we find that there is a tight link between district-level popular support for free-trade and subsequent support for PR. A ten percentage point increase in support for the protectionist general tariff is associated with a five percentage point decrease in support for PR. This linkage persists to the 1918 adoption of PR. Furthermore, we employ a synthetic control approach to assess if the introduction of PR in 1918 dampened the growth of protectionism in the interwar period Our results have important implications for theories of endogenous political institutions.