This article provides a novel perspective on party organization by showing how the disloyalty of political brokers causes party fragility. Brokers are often essential connecting parties to voters. In broker–mediated party systems, however, brokers may be unreliable agents, regularly changing political allegiances in search of better returns for their brokerage. This free agency from brokers hinders durable party-voter linkages. However, measuring how brokers influence parties is empirically complex. Taking advantage of the fact that these agents are also local candidates in Brazil, this article is the first to show on a national scale the electoral consequences of brokers’ free agency on party future performance. Natural experiments and an unexpected, temporary institutional reform that raised exit costs for a group of brokers reveal that when party switching is costly, brokers become dependable party intermediaries, dramatically improving the electoral prospects of their parties.