The Defense of Marriage Act is an exception to the rule that a law’s title is as uninformative about the law’s purpose as the titles of Marx Brothers movies (“Duck Soup,” “Animal Crackers”) are about those movies’ contents. DOMA’s purpose is precisely what its title says. Which is why many conservatives and liberals should be uneasy Wednesday when the Supreme Court hears arguments about DOMA’s constitutionality.

Conservatives who supported DOMA should, after 17 years’ reflection, want the act overturned because its purpose is constitutionally improper. Liberals who want the act struck down should be discomfited by the reason the court should give when doing this.

DOMA, passed by Congress in 1996, defines marriage for the purpose of federal law as a legal union between one man and one woman.

Because about 1,100 federal laws pertain to marriage, DOMA’s defenders argue that Congress merely exercised its power to define a term used in many statutes. But before 1996, federal statutes functioned without this definition, which obviously was adopted for the “defense” of marriage against state policies involving a different definition. “Before DOMA,” an amicus brief submitted by a group of federalism scholars notes, “federal law took state law as it found it.”

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 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 the question of 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”?

DOMA’s obvious purpose is, as the scholars’ brief says, “to reject state governments’ policy judgments.” Its purpose is to endorse, and to some extent enforce, the traditional understanding of marriage. The scholars’ brief says:

“Congress may regulate in this area to the extent necessary to further its enumerated powers. But it may not simply reject the states’ policy judgments as if it had the same authority to make domestic-relations law as they do. That is the difference between a government with a general police power and a government of limited and enumerated powers.”

Federalism enables diversity as an alternative to a congressionally imposed, continent-wide moral uniformity. Letting Washington 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’ usurpation of states’ powers — the court would defer to 50 state governments, including the 38 that today ban same-sex marriage.

Liberals praise diversity but generally urge courts to permissively construe the Constitution in order to validate federal power to impose continental uniformities. DOMA is such an imposition. Liberals may be rescued from it by jurisprudence true to conservative principles, properly understood.

George Will is a columnist for The Washington Post. He can be contacted at:

georgewill@washpost.com