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A pitched battle is being fought that could signal federalism’s future -- and it has nothing to do with health-care reform. Rather, states and the U.S. House of Representatives are arguing the validity of a 1996 law that defines marriage as exclusively between a man and a woman.

The latest flashpoint in the debate, currently being heard in a federal appeals court, began with a 2010 lawsuit. New York resident Edith Schlain Windsor, who owed the IRS more than $363,000 in estate taxes after her wife died, sued the feds, arguing that she shouldn’t be subject to different tax laws than other married couples just because her marriage was to a woman.

Historically, when a spouse dies, the estate tax isn’t triggered, and the survivor collects an inheritance tax-free. But while New York recognizes Windsor’s same-sex marriage, the federal government does not, thanks to the Defense of Marriage Act (DOMA). Because federal law doesn’t view same-sex marriages the same way it views heterosexual marriages, the unlimited marital tax deduction doesn’t apply to Windsor.

At issue, her attorneys argue, is that the federal government should defer to states when determining whether a couple is validly married.

In June, U.S. District Judge Barbara Jones agreed with Windsor and ruled that DOMA was unconstitutional. By sanctioning some states’ definition of marriage and rejecting others, Jones wrote, the federal government had launched “a sweeping federal review” that “does not square with our federalist system of government, which places matters at the ‘core’ of the domestic relations law exclusively within the province of the states.”

Since February 2011, the Obama administration has refused to defend DOMA, so a body within the U.S. House has taken up its defense and is appealing. Democratic attorneys general in Vermont, New York and Connecticut, which make up the Second Circuit where the case is on appeal, filed an amicus brief this fall in support of Windsor. They argue that DOMA serves little purpose other than to stigmatize same-sex couples and “significantly intrude on core state powers.”

Meanwhile, 14 Republican attorneys general filed their own amicus brief defending DOMA -- an unusual stance for red state public officials who often decry federal overreach into state matters. They argue that DOMA encourages procreation. They also fear that if a court finds the law unconstitutional on equal protection grounds, it could invalidate similar state-level statutes.

DOMA has been found unconstitutional in other federal cases, and most Supreme Court observers think the High Court will probably rule on same-sex marriage soon. It’s less clear how the court will take it up. Some speculate that it may address same-sex marriage more directly and weigh in on challenges to California’s Proposition 8, the 2008 ballot initiative that bans same-sex marriage.