A growing experimental literature suggests that international law appears to have a larger impact on public opinion than constitutional law. We develop a theory to explain these seemingly counter-intuitive results. First, in studies that explore the impact of constitutional law, respondents may be “unsuccessfully treated.” Second, constitutional law treatments can trigger a backlash effect through defensive processing of information on constitutionality. To test our theory, we simultaneously test the normative effect of international law and law in relation to the policy of separating migrant families at the U.S. border. We fielded nationwide survey experiments in July 2018 and November 2020, asking respondents whether they supported the policy. Consistent with our theory, we find that telling people that the policy is unconstitutional increases support for the policy, but only when the issue was receiving heavy media coverage, and that international law has no comparable effect on public opinion.