Factionalism is an important element in parties' internal life, affecting their policies, strategies, electoral performance and even survival. Yet little is known about how parties manage factionalism. This study examines how radical left parties (RLPs) manage factionalism, drawing on a comparative case study of four RLPs – the Spanish Podemos and United Left (IU), and the Portuguese Left Bloc (BE) and Communist Party (PCP) – from 2010 to 2019. Drawing on original data collection, we find that parties adopt both formal and informal mechanisms to address factionalism. However, their approaches differ significantly. We identify two main approaches towards factionalism: a permissive approach, which allows internal pluralism, in the BE and IU; and a prohibitionist approach, which actively fights factionalism, in Podemos and the PCP, with competitive prohibitionism in the former and pre-emptive prohibitionism in the latter. These approaches strongly correlate with parties' origins and political orientation, but neither fully prevents intra-party conflict or splintering.

Who are the supporters of former rebel parties? Drawing on the classical party literature, we argue that the support base of former rebel parties consists predominantly of activists. These supporters are dissatisfied with intra-party politics because the benefits and costs of a rebel-to-party transformation are unequally distributed between them and the leadership. We test our arguments by examining the case of the novel FARC party (Comunes) in Colombia. Based on a unique sample of FARC supporters, we obtained unprecedented insight into the internal affairs of the FARC. Our survey design allowed us to generate an over-time comparison between FARC and other party supporters in the Colombian political system. The empirical analysis shows that in comparison, FARC supporters tend to be more politically active as well as more dissatisfied with internal politics. Furthermore, dissatisfaction with democracy and the peace agreement increased after their first electoral cycle.

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.