Abstract This article explains the twentieth-century Latin American shift from majoritarian to proportional representation (PR) electoral systems. It argues that PR was introduced when the electoral arena changed significantly and threatened the power of the dominant party. The adoption of PR was therefore an effort by the established party to retain partial power in the face of absolute defeat. Majoritarian systems remained in place when the incumbent party was strong enough to believe that it could gain a plurality of the votes despite electoral changes. An empirical analysis of 20 countries over 104 years (1900–2004) provides support for this argument.