The question now is whether DOMA is “necessary and proper” for the exercise of a constitutionally enumerated congressional power. There is no such power pertaining to marriage. This subject is a state responsibility, a tradition established and validated by what can be called constitutional silence: The 10th Amendment says, “The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.”

The amicus brief takes no position on same-sex marriage as social policy. Rather, it addresses a question that should obviate the need to address whether DOMA violates the constitutional guarantee of equal protection of the laws. The threshold question is: Does the federal government have the power that DOMA’s preamble proclaims, the power “to define and protect the institution of marriage”?…

As the scholars’ brief says, DOMA “shatters two centuries of federal practice” by creating “a blanket federal marital status that exists independent of states’ family-status determinations.” Federalism, properly respected, enables diversity as an alternative to a congressionally imposed, continent-wide moral uniformity. Allowing Washington to impose such conformity would ratify unprecedented federal supremacy regarding domestic relations, a power without judicially administrable limits. By striking down DOMA — by refusing to defer to Congress’s usurpation of states’ powers — the court would defer to 50 state governments, including the 38 that today prohibit same-sex marriage.