The Constitution requires that all legislators, judges, and executive officers swear or affirm their fidelity to it. The resulting practice, often called “the oath,” has had a pervasive role in constitutional law, giving rise to an underappreciated tradition of promissory constitutionalism. For example, the Supreme Court has cited the oath as a reason to invalidate statutes, or sustain them; to respect state courts, or override them; and to follow precedents, or overrule them. Meanwhile, commentators contend that the oath demands particular interpretive methods, such as originalism, or particular distributions of interpretive authority, such as departmentalism. This Article provides a new framework for understanding the oath, its moral cont...