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In latent scaling applications, such as the positioning of political parties, differential item functioning (DIF) may occur because of measurement issues or because of substantive differences in the association between latent and manifest variables. While the first source of DIF has received considerable attention, the second has not, although it is of potential interest to comparative scholars. In this research note, we introduce a novel hierarchical Bayesian item response model that allows us to disentangle different sources of DIF. Drawing on the 2019 Chapel Hill Expert Survey (CHES), we highlight how the same issues are unequally politicized across Western Europe, and how some issues are less ideologically determined than others. Our model can be adapted to alternate settings, allowing researchers to shine a light on variation in, e.g., ideology, issue politicization, or party competition.

Western Europe is experiencing growing levels of political polarization between parties of the New Left and the Far Right. In many countries, the socio-structural foundations of this divide (class, education, residence) are by now so clear that many interpret this divide as a fully mobilized new electoral cleavage. At the same time, observers have highlighted a growing fragmentation of party systems and the proliferation of new competitors. We suggest to make sense of these contradic- tory developments by focusing on the shared group identities that constitute the “glue” of cleavage formation translating grievances into political antagonisms. Our contribution relies on data from an original online survey fielded in France, Germany, the UK and Switzerland. Respondents an- swered questions on their sense of belonging to a series of social groups, electoral preferences and socio-demographics. On this basis, we are able to show – observationally – that socio-structural categories relate to both socio-economically (e.g. class) and socio-culturally (e.g. cosmopolitanism, lifestyle) connoted group identities, which divide New Left and Far Right voters in surprisingly similar ways across contexts. We then study the extent of social closure and political mobilization at the extremes of the new divide through the analysis of social networks, perceived group align- ments, and perceived representation. Our findings suggest that the new conflict is firmly rooted in socio-economic categories and at the societal level. Its political mobilization happens mostly via culturally connoted identities. What is more, social realignments and closure are highly similar across the four countries. This underscores that party competition remains rooted in structural an- tagonisms even as support for individual parties becomes more volatile. To detect this underlying similarity and stability, it is necessary to focus on voter alignments to ideological party blocs, rather than individual parties