Switching parties is arguably the most important decision a politician will ever make. This book is the first-ever systematic study of the causes and consequences of legislative party switching in the United States. The author argues that re-election alone does not explain party switching. He proposes an ambition-based theory that accounts for multiple goals (including higher office aspirations and the desire for influence in the legislature) with a focus on the electoral costs and the institutional benefits of the decision. The book combines the statistical analysis of electoral data and legislative careers in the US Congress and state legislatures with elite interviews of party switchers, non-switchers, and a party leader. The case study of a party switcher's decision in 'real time' documents the complexity of the decision in a politician's own words prior to and following the switch. The book raises important questions regarding the meaning of a party label.