On Thursday, a three-judge panel of the U.S. Court of Appeals for the Second Circuit was the latest to issue a ruling on the constitutionality of the Defense of Marriage Act (DOMA). Over a strong dissent by one of the judges, it ruled that section 3 violates equal protection and thus is unconstitutional.

Section 3 defines “marriage” for the purpose of federal law as a “legal union between one man and one woman” and “spouse” as referring “only to a person of the opposite sex who is a husband or a wife.” The challenge in Windsor v. United States was brought by a surviving spouse of a same-sex couple, married in Canada and residing in New York, who was denied the benefit of a spousal deduction under federal tax law.

This case is one of about half a dozen challenges in which the federal government originally defended the constitutionality of DOMA, which was passed by large majorities in both houses and signed into law by President Clinton in 1996. Following an announcement by Attorney General Eric Holder in 2011 that the Administration would no longer defend DOMA, the Bipartisan Legal Advisory Group of the U.S. House of Representatives hired special counsel to defend the statute in court.

Two aspects of these challenges bear mentioning. First, there is fundamental disagreement among the courts about whether DOMA is subject to rational basis review or intermediate scrutiny. Second, nothing in DOMA prevents states from defining marriage as they see fit for purposes of state law. Ironically, champions of same-sex marriage argue that striking down DOMA is a matter of federalism. As Ed Whelan from the Ethics and Public Policy Center pointed out:

Properly understood, federalism leaves state matters to the states and national matters to the national government. The only genuine interest of federalism at stake…is the ability of Congress to determine what marriage is for purposes of federal law.

The principal issue courts are grappling with is whether the classification in DOMA (here, defining marriage for the purpose of federal law as a “legal union between one man and one woman”) is subject to a rational basis standard of review, intermediate scrutiny or strict scrutiny (although no court has held that classifications based on gender preference are subject to the latter), and whether the proffered justifications for DOMA pass muster under those standards.

There are three levels of review for an equal protection challenge of this kind: (1) strict scrutiny, which is reserved for classifications based on race, religion or national origin, and which Justice Lewis Powell famously called “strict in theory, but fatal in fact” since nearly all laws subject to strict scrutiny have been struck down; (2) intermediate or heightened scrutiny, which has been applied to, among other things, classifications based on gender and illegitimacy; and (3) rational basis review, which has been applied to all other classifications such as age or income. When a court reviews a classification subject to strict scrutiny, the government must demonstrate a compelling need and that the provision is narrowly tailored to achieve that objective. When a court reviews a classification subject to intermediate scrutiny, the government must demonstrate that the classification is substantially related to an important government objective. When a court reviews a classification subject to a rational basis inquiry, the government need only demonstrate that the classification bears a rational relationship to an appropriate governmental interest.

In defending DOMA, the Bipartisan Legal Advisory Group asserts that the government interests at stake include, among others, preserving a uniform federal definition of marriage for purposes of obtaining federal benefits, protecting the federal fisc, and avoiding “the unknown consequences of a novel redefinition of a foundational social institution.” It also argues that Congress passed DOMA to encourage “responsible procreation.”

Some of these cases are currently pending before the Supreme Court, and others are making their way there quickly. The Supreme Court has not acted on any of these petitions yet, but given the fact that several courts have now struck down a federal statute on constitutional grounds, it is almost certain that the justices will hear one or more of these cases this term.