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Why are American politicians “single-minded seekers of reelection” in some decades and fierce ideological warriors in others? This article argues that the key to understanding the behavior of members inside a legislative chamber is to follow the actions of key figures outside the chamber. These outsiders—activists, interest groups, and party bosses—use their control over party nominations, conditioned on institutional rules, to ensure ideological behavior among officeholders. To understand how vital these outsiders are to legislative partisanship, this article takes advantage of a particular natural experiment: the state of California's experience with cross-filing (1914–59), under which institutional rules prevented outsiders from influencing party nominations. Under cross-filing, legislative partisanship collapsed, demonstrating that incumbents tend to prefer nonpartisanship or fake partisanship to actual ideological combat. Partisanship quickly returned once these outsiders could again dominate nominations. Several other historical examples reveal extralegislative actors exerting considerably greater influence over members' voting behavior than intralegislative party institutions did. These results suggest that candidates and legislators are the agents of activists and others who coordinate at the community level to control party nominations.