This paper analyses the dynamics between ethnic boundaries and electoral alignment in the context of western minority nationalisms by focusing on the Catalan case. In particular, the research explores the changes in boundary shifts at the electoral level, whether they affect differently pro-sovereignty and pro-union parties, and to what extent changes have reinforced the ethnic alignment of vote. Methodologically, the analysis is based on observational data from the elections of 2010 and 2012, which allows control over some of the traditional limits of opinion studies. The results suggest that ethnicity is a dynamic factor that has gained relevance for both sub-state and state-wide parties, and that processes of boundary contraction are not necessarily associated with electoral failure.