Why do elites extend the franchise? When do they prefer proportional representation? Classical explanations lie in modernisation, the threat of revolution, and the electoral rise of socialist parties. A complementary explanation points at intra-elite pacts, and intra-party struggles over control of the legislative agenda of democratisation. Nineteenth century Belgium was the first country to adopt a proportional electoral system for national elections. In this crucial case, we study how extra-parliamentary threats and intra-party competition affected the legislative processes that first led to the adoption of universal suffrage tempered by plural voting, and subsequently to the introduction of PR. A longstanding puzzle is why Belgian conservative elites opened up the electoral process while they held sizeable majorities in parliament. Our paper’s contribution is to clarify the timing and the relative importance of existing explanations for this puzzle by providing the first micro-evidence of dozens of roll call votes at the legislator level. Inter- and intra-party struggles over the extension of the franchise crucially help to understand why proportional representation was introduced.