This article begins with a study of the ways in which France’s market for supplemental social insurance for local government officials has been restructured. It proceeds to discuss the political effort of the country’s leading supplemental mutual insurance company to maintain its monopoly. The article shows that the operator in question, which has historically been the leading provider of supplemental health and life insurance for local public-sector workers, mobilized local employers, administrative managers, and public policymakers in political coalitions. The operator succeeded in maintaining control of its market relationships and defending its preferred reform measures by framing its arguments in moralistic terms, affirming its identity as an affinity group insurer, and building institutional alliances.