What explains the development of legislative party voting unity? Evidence from the United States and Britain indicate that partisan sorting, cohort replacement effects, electoral incentives, and agenda control contributed to enhancing party cohesion during the nineteenth and twentieth centuries. Here, these mechanisms are evaluated by analysing a dataset containing all the recorded votes from the Canadian House of Commons, 1867–2011. Overall, we find that partisan sorting and the government’s ability to control the agenda are central to the consolidation of parties over time. Our results underscore the need to integrate institutional rules and legislative agendas into models of parliamentary voting behaviour and suggest that strict party discipline can lead to the development of a multi-party system in the legislative arena.