This chapter presents a unified model of legislative elections, parties, and roll call voting, built around a party leadership election. First, a legislative caucus selects a party leader who campaigns based on a platform of a disciplinary system. Once elected, that leader runs the legislative session, in which roll call votes occur. Then elections occur, and incumbents face re-election with the positions they incrementally adopted. When the caucus is ideologically homogeneous, electorally diverse, and policy motivated, members will elect a leader who solves the collective action problem of sincere voting with “preference-preserving influence.” That leader will threaten to punish legislators who bow to electoral pressure to vote as centrists. Consequently, legislators vote sincerely as extremists and get slightly lower vote shares, but they offset that lost utility with policy gains that they couldn’t have gotten without party influence. Party leaders will rarely pressure legislators to vote insincerely.
