Electoral reform creates new strategic coordination incentives for voters and elites, but endogeneity problems make such effects hard to identify. This article addresses this issue by investigating an extraordinary dataset, from the introduction of proportional representation (PR) in Norway in 1919, which permits the measurement of parties’ vote shares in pre-reform single-member districts and in the same geographic units in the post-reform multi-member districts. The electoral reform had an immediate effect on the fragmentation of the party system, due in part to strategic party entry. The authors find, though, that another main effect of the reform was that many voters switched between existing parties, particularly between the Liberals and Conservatives, as the incentives for these voters to coordinate against Labor were removed by the introduction of PR.
