Research increasingly characterises politics as a “mega-identity” capable of encapsulating other social divisions, yet empirical tests of how this identity meddles and absorbs other social cleavages remain rare, particularly in multiparty systems. This study addresses this challenge by developing a political Coherence Index, a continuous metric designed to quantify the structural alignment between a respondent’s attributes (across 12 social dimensions) and the prototypical values of their ideological group. Using data from a stratified national survey in Spain (June 2024, N = 3,015), we first confirm the "primacy of partyism": political divides generate significantly larger affective gaps than any other social cleavage, including religion, gender, or region. Additionally, we also show that attitudes towards a varied range of 26 social groups cluster along ideological lines. Crucially, we then demonstrate that partisan feelings are driven by the consolidation of a political mega-identity. We find that as individuals become more "coherent"—meaning their social identities and values increasingly match their party’s prototype—their affective distance from outgroups widens. This effect is symmetrical across the ideological spectrum, operating with equal force for both left- and right-wing partisans. These findings suggest that affective polarisation is not merely a matter of partisan preference, but a product of how intensely partisan identity aligns with one’s attributes, values and social spheres.