Party switching and policy disagreement: scaling analysis of experts' judgment

Abstract Under what conditions do policy disagreements cause parties to split? Before the general elections in 2017 in Japan, the Minshin, the largest opposition party, split, and the major opposition camp collapsed into three parties. Disagreement about defense policy was rapidly politicized by the founding leader of a new party, Hope. Some members were motivated to switch to Hope and split the Minshin. Why did the disagreement about defense policy, which had long existed but was inactive, become an issue that led to a major split? To answer these questions, we used Aldrich–Mckelvey and Blackbox transpose scalings to analyze the data from an expert survey about issue positions and the salience of party policies. The variance in experts' responses was also examined to consider whether parties consistently and clearly distinguish their policy positions. Party positions on defense policy were clearly distinguished and revealed a split among splinter parties. However, the party positions clearly converged on the environment, decentralization, and other policies in a two-dimensional space, which indicated the unity of the major opposition camp. Moreover, different item functioning (DIF) analysis revealed a high variance (i.e., negative weight) on the issue salience of defense policy, which indicated its politicization immediately before the split. Overall results indicated that the politicization of defense policy contributed to weakening party cohesion and the party was split over disagreement about it. This implies that party switching is caused by policy disagreement but also hinges critically on which policy is politicized as an issue for party unity.