Even if the non-unitary nature of parties has come back into the party politics agenda, many of its features are still largely understudied. Specifically, an encompassing explanation of individual faction membership and of the role of party fusions in fostering faction membership is still missing. By performing a diachronic analysis, this article proposes a new approach to study the determinants of faction membership, highlighting the fundamental role of ideological, policy- and career-related factors. Moreover, the article uses as an explanatory factor a key element that has hitherto never been taken into account in intra-party analyses: psychological social identity, a variable that strongly conditions party members’ behaviour in situations where parties are merging. The analysis also shows the crucial role of party fusion in shaping individual faction membership determinants, highlighting that the effect of these determinants varies considerably the more time has elapsed since the party’s merger.
