Abstract: Boundaries, policies that reinforce unequal life chances through sorting for opportunity, are persistent in rich democracies despite reform efforts. However, the political resilience of policies creating boundaries differ greatly both within reform episodes and across countries. To understand why, this paper analyzes three case studies of attempts at reforming opportunity boundaries in a diverse set of rich democracies: metropolitan-scale desegregation efforts in the US, school tracking reform in the German city-state of Hamburg, and attempts to loosen dismissal rules for regular workers in Japan. Introducing a novel comparative theory, this paper argues that context-specific interactions between credit, education, and labor institutions mean only certain boundaries are adaptively relied upon by marginal middle class families to secure opportunity through exclusion in the absence of viable alternative strategies. Applied to the case studies, the theory explains why electorally pivotal mass backlash specifically preserved city-suburban district boundaries in the US North, early-age school tracking boundaries in Germany, and dualized employment protection boundaries in Japan. Within each case study, evidence from a mixed-methods analysis of public opinion, election outcomes, and policymakers' decision-making supports this explanation for why reforms targeting these specific policies were blocked by backlash, while others in the same episodes were successfully reformed despite previous political resilience. Taken together comparatively, the case studies suggest that majority interests conditioned on institutions complement elite-driven or policy-specific explanations for reform failure, that insider-outsider divides characterize opportunity in rich democracies beyond dualized employment protection, and that differences in skill formation cross-nationally partly reflect insecure voters' defense of boundaries that secure opportunity through exclusion rather than economic function alone.