Conventional wisdom holds that proportional representation (PR) was introduced in many European democracies by conservative parties seeking to preserve their legislative seat shares after franchise extension and industrialization increased the vote base of socialist parties. In contrast to this seat-centered account, we focus on how PR affected party leaders' control over nominations, thereby enabling them to discipline their followers, and facilitating the negotiations needed to form governments. We explore this portfolio-centered account with the case of Norway, using roll call data from six different reform proposals in 1919. We show that party leaders were more likely to vote in favor of adopting PR than rank-and-file members, even controlling for the parties' expected seat payoffs and the district-level socialist electoral threat facing individual legislators. Moreover, using within-legislator variation, we show that the internal cohesion of the major parties increased significantly after the introduction of PR.