District coordination versus party consolidation: alternative ways Duverger’s law works … sort of

Electoral reforms to first-past-the-post are often expected to consolidate party systems, but parties can instead coordinate without merging. We develop a partition-function framework distinguishing party consolidation from district-level coordination (“Duvergerian moves”), where allies strategically withdraw candidates across districts to maximize seats. We motivate the argument with the 2024 French legislative election, where coordinated withdrawals by NFP and Ensemble sharply reduced multi-candidate runoffs and helped block National Rally. We then study Poland’s Senate after the 2011 switch to single-member districts. Despite limited change in the effective number of parties, opposition parties progressively refined district coordination, culminating in near-complete one-candidate-per-district arrangements in 2019. We propose three coordination indices to measure the extent and effectiveness of such strategies, showing how coordination can reproduce majoritarian incentives while preserving party identities.