Legislative party unity is a fundamental building block of democratic politics. The value of party labels, parties’ bargaining power within the legislature, and, consequently, parties’ ability to implement their policy platforms depend on both their unity in legislative voting and their seat shares. Interestingly, party unity and party size might themselves be related. We examine several potential relationships between party size and party unity. While conventional wisdom suggests that unity is more difficult to maintain in large parties than in small ones, we argue that this assertion holds only if parties care solely about policy implementation. If parties also care about the value of the party label, two alternative hypotheses are viable. First, there is the null hypothesis of no relationship between party size and party unity. Second, since party discipline to impose unity might be “cheap” in parties with large majorities, we may expect party cohesion to increase with party size. We test these three possibilities using legislative roll-call data from ten European democracies, taking into account as well the potential effects of majority or minority status, opposition-party unity, and the electoral system.
