Factional competition within parties is pervasive and plays a crucial role in shaping their internal cohesion and overall political strength. Much like interest groups, factions seek to represent specific societal interests and employ both internal and external strategies to pressure actors towards preferred policy positions—often beyond researchers’ observability. In this paper, we combine interest group and party politics research to argue that factions make strategic choices between ‘privatizing’ and ‘socializing’ disagreement depending on the internal opportunity structures of their parties. When parties have inclusive internal structures, factions are more likely to use internal channels—such as congress motions, committee participation, or leadership bids—to achieve influence. When intra-party structures are more exclusive, factions instead rely more on external strategies, including media appearances and social media advocacy. To test these hypotheses, we designed a multi-modal sampling frame strategy combining top-down and bottom-up data collection. As a first step, we systematically map and identify factional actors through large-scale extraction and classification of organizational information from party sources and the analysis of newspaper coverage to detect publicly visible factional activity. Building on this mapping, we subsequently surveyed, for the first time, all identifiable potential factions operating in 25 parties in Germany, the Netherlands, and the United Kingdom (N > 5,000). The project bridges the literature on intra-party politics and interest group behaviour, contributing new empirical insights into the conditions under which factions choose particular influence strategies and the extent to which these strategies are shaped by party organizational structures.