Historical, behavioral, and institutional research all provide convincing evidence about the important role of political parties in the American political system. Parties are a valuable resource for candidates seeking office and help current officeholders achieve their electoral and policy goals. Given parties’ significance inside and outside of government, understanding why an elected official would change parties is both theoretically and substantively important. In Crossing the Aisle, Antoine Yoshinaka uses ambition theory to develop a compelling explanation for why legislators switch parties.

Although ambition theory has been more commonly used to explain candidate emergence and progressive ambition, Yoshinaka is not the first to note its utility in explaining politicians’ calculus regarding partisan affiliation. In Why Parties?, John Aldrich offers a powerful demonstration of how ambition theory can help explain a legislator’s decision to join one party over another. Yoshinaka expands on earlier work and demonstrates how ambition theory can explain not only why some legislators switch parties but also why many cross-pressured members do not. The latter is particularly important because, as Yoshinaka notes, scholars can learn a great deal from investigating why a political phenomenon did not occur.

Indeed, one