One important criterion for assessing the quality of democratic governance is the extent to which the policy process effectively translates citizen preferences into collective choices. Several scholars have observed a discrepancy between citizen preferences for strong environmental protection and weak policies adopted in the United States, indicating that the United States may fall short on this criterion. We examine one possible mechanism contributing to this discrepancy—legislator defection from campaign promises. Our data indicate that legislators in the U.S. Congress routinely defect from their campaign promises in environmental protection, undermining the link between citizen preferences and policy choice. We also find that legislators are much more likely to defect from pro-environmental campaign promises, which moves government policy toward less stringent environmental programs. Finally, the propensity of legislators to defect from their campaign promises is systematic, with defection affected by partisanship, constituency influence, the influence of the majority party, and the likely consequences of defection for policy choice. These findings contribute empirical evidence relevant to the “mandate theory” perspective on how citizen preferences are translated into collective choices through the policy process. These findings may also complement research in comparative politics concluding that legislatures selected through single member districts adopt less stringent environmental policies than do legislatures chosen via proportional representation in that the mechanism for this effect may go through legislator defection from campaign promises.