The erosion of constitutional tolerance

Abstract Constitutional tolerance—the voluntary subordination of member states to EU law—was once the normative hallmark of the European Union’s constitutional order. This hallmark is now eroding: subordination of member states to EU law is in question, and so is its voluntary character. Against this backdrop, the article revisits a long-neglected question: what are the foundations of constitutional tolerance in Europe? Drawing on federal theory, it shows that constitutional tolerance depends on a precarious federal balance between contradictory commitments to unity and diversity. The article argues that the Court of Justice of the European Union is developing article 2 of the Treaty on European Union (TEU) into a judicially enforceable guarantee clause as a means of governing this tension through the defense of a constitutional project shared by the member states. However, this approach is associated with an often-overlooked challenge in the European Union: the member states are characterized by different varieties of constitutionalism, with radically different conceptions of the constitutional ideas set out in article 2 TEU, including democracy and the rule of law. While the judicial enforcement of shared values may serve as a mechanism of constitutional defense, it also risks displacing the federal balance on which constitutional tolerance ultimately depends.