We present an intra-party account of electoral reform, contrasting the incentives of legislators (MPs) with those of party leaders. We develop our argument along the switch to proportional representation (PR) in early 20th century Europe. District-level electoral alliances allowed bourgeois MPs to counter the “socialist threat” under the electoral systems in place. PR was thus unnecessary from the seat-maximizing perspective that dominates previous accounts—intra-party considerations were crucial: candidate nomination and legislative cohesion. We show our argument to hold empirically both for the prototypical case of Germany, 1890–1920, using encompassing district-level data on candidatures, elections, electoral alliances, roll call votes and a series of simulations on reform effects; and for the implementation of electoral reforms in 29 countries, 1900–31.