WASHINGTON – A law denying gay married couples the federal benefits that their straight counterparts receive appears to be in deep trouble at the U.S. Supreme Court.

A majority of the nine justices expressed deep skepticism Wednesday about the constitutionality of the 1996 Defense of Marriage Act, which defines marriage on the federal level as being between a man and a woman only.

During a two-hour oral argument, Justice Anthony M. Kennedy – the swing vote on most closely divided cases – stressed that the Constitution leaves laws concerning marriage to the states, meaning any federal law defining marriage raises concerns about the federal government improperly usurping state power.

Noting that married couples enjoy 1,100 federal benefits – which the act denies to gay married couples – Kennedy said that under the Defense of Marriage Act, “you are at real risk of running in conflict with what has always been thought to be the essence of the state police power, which is to regulate marriage, divorce, custody.”

Meanwhile, the court's four liberal justices indicated they have grave concerns about whether the Defense of Marriage Act violates the Constitution's guarantee of equal protection under the law.

Under the law, “there are two kinds of marriage,” Justice Ruth Bader Ginsburg said. “Full marriage and the skim-milk marriage.”

Coming a day after the justices appeared wary, in a separate case, of making gay marriage the law of the land, Wednesday's oral argument hinted at a possible separate outcome: one in which states are allowed to decide whether to sanction gay marriages, but where the federal government would have to recognize gay marriages from states that allow them.

Such a ruling would have profound benefits for the gay couples in New York, which legalized gay marriage in 2011, and the other states that sanction such unions.

That's because those gay couples are now denied all sorts of federal benefits, of which Ginsburg highlighted just a few: “No joint tax return, no marital deduction, no Social Security benefits; your spouse is very sick but you can't get leave.”

Gay widows and widowers are also subject to the federal estate tax, which is what produced U.S. v. Windsor, the case the justices heard Wednesday.

Edith Windsor, of New York, married her partner of 50 years, Thea Spyer, in Ontario in 2007. Two years later, Spyer died – and the federal government collected more than $300,000 in inheritance taxes that Windsor would not have had to pay if Spyer had been a man.

In a nation founded on the notion of equal rights, that was profoundly unjust, argued Roberta A. Kaplan, a lawyer for Windsor.

“No one has identified in this case ... any legitimate difference between married gay couples on the one hand and straight married couples on the other that can possibly explain the sweeping, undifferentiated and categorical discrimination of DOMA,” Kaplan said.

Kaplan appeared to find an ally in Justice Elena Kagan, who cited a 1996 House report accompanying the Defense of Marriage Act, saying it was intended to “express moral disapproval of homosexuality.”

That being the case, Kagan asked: “Do we really think Congress was doing this for uniformity reasons, or do we think the Congress's judgment was infected by dislike, by animus, by fear?”

Defending the law on behalf of a bipartisan group of federal lawmakers who support DOMA, attorney Paul D. Clement acknowledged what the House report said.

“I think if that's enough to invalidate the statute you should invalidate the statute,” Clement said. “But that's never been enough.”

Clement conceded that some of the lawmakers who approved DOMA may have been motivated by anti-gay bias but said that doesn't mean some of them didn't have legitimate concerns as well.

Some may have been worried that “this is a redefinition of an age-old institution,” Clement said.

Noting Hawaii appeared in the mid-1990s to be poised to approve gay marriage, Clement put himself in the mind of the Congress of the time. He said Congress feared that people would travel to Hawaii to get married and then return home, where states would, by law and tradition, recognize that out-of state marriage.

“Then the federal government will borrow that definition, and therefore, by the operation of one state judiciary, same-sex marriage is basically going to be recognized throughout the country,” Clement said.

The court's conservative justices seemed subdued during much of the argument on the merits of the case but came to life during the section of the argument devoted to whether the court was properly considering the case in the first place.

Chief Justice John G. Roberts seemed annoyed at the unusual legal situation the Obama administration created when it defied the tradition that calls on the executive branch to defend all federal laws in court. On the contrary, Obama refused to defend DOMA.

That left the case in the hands of Clement, representing those federal lawmakers who continue to support the law, and Vicki C. Jackson, a court-appointed lawyer.

Roberts said it was bizarre for the administration to agree with an appeals court ruling overturning DOMA and yet ask the Supreme Court to hear the case and reaffirm the lower-court decision.

“You're asking us to do something we've never done before,” Roberts said.

Meanwhile, Justice Antonin Scalia said it was a “new world” in which the administration would oppose the law but continue to enforce it.

The justices also will have to decide whether the lawmakers who brought the case to the high court had the right to do so, but there seemed little interest in denying them that right.

“It seems to me there's injury here,” meaning the case is properly before the court, Kennedy said.

Still, when deciding the case – most likely in June – the court could rule that the plaintiffs do not have standing, thereby letting the lower court ruling overturning DOMA stand.

The justices also could decide they should not have taken the case in the first place, which would, in effect, also doom DOMA.

The court also could uphold the law or overturn it, and given Kennedy's comments, the latter option seems more likely.

The federal government should honor “the historic commitment of marriage and questions of the rights of children to the states,” said Kennedy, a traditional proponent of state's rights and the author of the high court's two landmark gay rights decisions.

Addressing reporters outside the court about the case that bears her name, Windsor, 83, lamented the loss of her spouse and said that even though they had been together for four decades, their marriage made their relationship feel like “magic.”

The Supreme Court argument went “beautifully,” she said, expressing optimism about the case's outcome.

“Today is like a spectacular event for me,” Windsor said. “And I know that the spirit of my late spouse, Thea Spyer, is right here watching and listening and would be very proud and happy of where we've come to.”