Constitutional pluralism has long been controversial, but has recently come under renewed attack. Critics allege that by justifying departure by national courts from the Court of Justice of the European Union’s (CJEU) orthodoxy on the primacy of EU law, constitutional pluralism is (at best) susceptible to abuse by autocratic member state governments, or is (at worst) a favored tool of Europe’s new authoritarians. This article is a defense of heterodoxy in European constitutional thought, and of constitutional pluralism in particular. It uses the concept of “loyal opposition” as a framing discourse, allowing us to see that heterodox approaches to EU constitutionalism and opposition to the received and dominant interpretation of the primacy of EU law are not necessarily any less “loyal” to the principles and values of European integration than agreement with the CJEU. A “legitimacy test” is proposed, by which we can determine whether a given instance of national judicial disagreement with the CJEU is loyal, principled opposition, or disloyal, abusive opposition.