Candidate coalitions and strategic manipulation in multi-winner elections: An evaluation of committee scoring rules

Strategic candidacy in multi-winner elections describes situations where candidates may enter or withdraw to influence which candidates are elected. Existing studies typically assume all eligible candidates are interested in participating. In reality, some eligible candidates are uninterested in winning but may enter strategically, while interested candidates may strategically withdraw; both types of moves can change the set of elected candidates. This paper extends the standard framework by distinguishing interested and uninterested candidates and studying manipulation via coordinated entry and exit. We focus on evaluating whether multi-winner rules are immune to manipulation by coalitions of candidates with respect to the number of voters n and the committee size k. Specifically, we say that a rule satisfies candidate-coalition strategy-proofness (CCSP) for an election-point(n,k) if the “true” strategy profile—where candidates enter the election if and only if they are interested—constitutes a strong Nash equilibrium (i.e., no coalition of players can achieve a collective benefit by deviating from the actions dictated by their true types). Focusing on five committee-scoring rules—k-Borda, k-Anti-plurality, BLOC, k-Plurality, and L1-CC—we provide complete characterizations of the election-points (n,k) where each rule satisfies CCSP. Specifically, k-Plurality is CCSP exactly when n<2k+2; L1-CC exactly when n Suggested Citation Bahloul, Badr & Doghmi, Ahmed, 2026. "Candidate coalitions and strategic manipulation in multi-winner elections: An evaluation of committee scoring rules," Mathematical Social Sciences, Elsevier, vol. 140(C). Handle: RePEc:eee:matsoc:v:140:y:2026:i:c:s0165489626000119 DOI: 10.1016/j.mathsocsci.2026.102504 as HTML HTML with abstract plain text plain text with abstract BibTeX RIS (EndNote, RefMan, ProCite) ReDIF JSON