Who’s the Real Rival? Intra- Vs. Inter-Party Margins Under Open List Proportional Representation

In open-list proportional representation (OLPR) systems, candidates often find their most formidable rivals not across party lines, but within their own party. This tension between intra-party and inter-party competition shapes how campaigns are waged and raises core questions of electoral design. We develop both probabilistic models and empirical analyses using data from OLPR countries to compare the margins of intra-party and inter-party victory. Using those models, we show that in many plausible institutional settings, intra-party threats dominate inter-party competition, challenging assumptions about party list teamwork under OLPR. Our focus is on electoral engineering: how institutional parameters such as the PR formula, district magnitude, and OLPR thresholds condition the balance between internal and external competition? The results highlight how institutional choices can tilt incentives toward intra-party rivalry rather than party cohesion, with significant implications for representation and party strategy.