From contagion to polarization

Abstract Why do mainstream parties respond differently to the emergence of radical challenger parties? Existing research argues that the electoral success of niche parties often induces mainstream parties to adopt more restrictive positions on immigration. However, little attention has been paid to how mainstream parties respond before challenger parties become electorally dominant or whether those responses necessarily produce policy convergence. This paper examines these questions through the case of Japan following the parliamentary emergence of Sanseitō in 2022 . Drawing on Meguid’s (2008) position, salience, and ownership (PSO) theory, Petrocik’s (1996) theory of issue ownership, and recent scholarship on political norm change, the study analyzes parliamentary debates, party documents, and the niversity of Tokyo–Asahi candidate survey covering thirteen election cycles from 2009 to 2026. The evidence reveals markedly asymmetric responses among Japan’s mainstream parties. The Liberal Democratic Party (LDP) shifted sharply toward more restrictive immigration positions during the 2024–2026 electoral cycle, whereas the Constitutional Democratic Party (CDP) continued to strengthen a rights-based and multicultural discourse. Sanseitō’s emergence coincided not with policy contagion but with widening ideological polarization between Japan’s major parties. The findings suggest that challenger parties can reshape political competition even without substantial electoral success. By reframing immigration from an administrative issue into one centered on national identity and cultural security, Sanseitō contributed to redefining elite political discourse. The Japanese case shows that challenger parties can deepen ideological polarization through asymmetric mainstream responses where convergence models would predict accommodation.