The constitutional structure of the federal government means the three branches often must interact during the policymaking process. Despite this need for interaction among the branches, few scholars have given attention to how the decisions of the Supreme Court may affect congressional policymaking, broadly defined. I argue that the Court's constitutional decisions can meaningfully impact congressional policymaking, both directly and indirectly. The Court's constitutional decisions not only determine the legitimate scope of congressional authority but also establish legal regimes within which members of Congress must act. During more restrictive legal regimes, members of Congress will be more likely to focus on and attempt to document evidence convincing the courts that a policy proposal meets existing constitutional standards. Furthermore, because congressional resources are scarce, legal regimes can also affect the allocation of congressional resources to policy proposals.I investigate hypotheses generated from my theory by examining bills introduced in the 101st. 109th Congresses that were based on congressional commerce authority or sought to strip states' of the immunity from federal lawsuits granted to them by the Eleventh Amendment. Using ordinary least squares regression, I examine the content of congressional committee hearings across legal regimes and the proportion of the hearing dedicated to establishing a policy proposal's constitutionality vis- -vis existing legal doctrine. I also examine the allocation of congressional resources across issue areas and legal regimes and employ logistic regression and duration/survival analysis to investigate the degree to which bills are treated differently in different legal regimes.The analyses support the conclusion that the Court's constitutional decisions affect legislative policymaking, but the nature of the effect is conditioned in part by the legal doctrine promulgated by the Court's decision.