While dislike of opposing parties, i.e., affective polarization, is a defining feature of contemporary politics, research on this topic largely centers on the United States. We introduce an approach that analyzes affective polarization between pairs of parties, bridging the US two-party system and multiparty systems in other democracies. Our analyses of survey data from 20 Western democracies demonstrate that affective polarization is closely linked with patterns of electoral representation: that is, who serves in office and electoral system proportionality. This comparative perspective highlights the structural underpinnings of affective polarization.