Protestors rally in support of gay marriage in front of the Supreme Court, March 27, 2013.(Reuters/Jonathan Ernst)

A majority of Supreme Court justices expressed concern Wednesday about a federal law that bars same sex marriage, signaling the nation's highest court may be poised to extend a potentially historic victory to the gay rights movement.

Hearing arguments in the second of two major gay marriage cases over two days, the probing questions from both wings of the court suggest the Defense of Marriage Act, which bars the federal government from recognizing same sex marriage in states where it is legal, could be struck down. Such a decision would be breakthrough for supporters of same sex nuptials, who until now have measured progress at a state by state level.

But the fundamental question of whether gay couples can marry in the U.S. will most likely not be settled by the court this time. Striking down DOMA would likely benefit only married same sex couples in the nine states (and the District of Columbia) where their marriages are legal. And a day earlier, justices hinted they might toss a challenge to Prop 8, a California law banning gay marriage, on procedural grounds.

Justices across the spectrum Wednesday seemed troubled by DOMA, which was passed by Congress and signed into law by President Bill Clinton in 1996. Clinton has since announced his opposition to the law.

The court's conservative leaning justices asked pointed questions about whether the law was anti-federalist, since it intrudes into states' traditional right to regulate marriage. The more liberal justices seemed amenable to the argument that DOMA discriminates against gay people and was passed with the clear intention of excluding an unpopular group.

Justices could strike down the law in a narrow way that would force the federal government to recognize same-sex marriages only in states where it's already allowed. There's a chance the court could rule more broadly, making dozens of state gay marriage bans legally vulnerable. Such an expansive ruling from the court is considered much less likely.

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The Justice Department would typically defend a federal law being challenged in the Supreme Court, but he Obama administration has declined to do so because it believes it is unconstitutional. Paul Clement, an attorney chosen by House Republicans who support DOMA, defended it instead.

Justice Anthony Kennedy, a conservative-leaning swing vote who has written two landmark opinions affirming gay rights, seemed unconvinced by the argument advanced by Clement that DOMA defines marriage as only between opposite-sex couples to avoid confusion. Clement said that the federal government has an interest in "uniformity," and had passed the law to avoid having to treat same-sex couples differently based on whether they live in states that allow gay marriage or not.

Kennedy pointed out that DOMA excludes married same-sex couples in more than 1,100 federal statutes and laws, which has a substantial impact on the "day to day life" of those couples and their children. He said the law does not provide uniformity because it affects "only one aspect of marriage."

Justice Ruth Bader Ginsburg said excluding married gay couples from sick leave, tax benefits, Social Security survivor benefits, and hundreds of other federal benefits and obligations, relegates same-sex couples to a "skim milk marriage" that is substantially worse than what heterosexual couples are allowed.

Justice Elena Kagan suggested that the law was not passed for uniformity's sake, but to discriminate. She read aloud from the House report on the law when it passed 17 years ago saying it expressed "moral disapproval of homosexuality."

Chief Justice John Roberts objected to the argument that Congress passed DOMA based on a dislike or hatred for gays and lesbians. He asked Attorney General Donald Verrilli, representing the Obama administration, whether he believed the 84 senators who voted for it at the time were all motivated by animus. Verrilli said the lawmakers could have voted for DOMA due to a "lack of careful reflection," but that the law discriminates no matter why it was passed.

Roberts also objected to Attorney Roberta Kaplan's characterization of gay people as a disadvantaged minority group lacking political power.

"As far as I can tell, political figures are falling over themselves to endorse your case," Roberts said.

But Roberts did seem concerned by the federalist argument. He, Kennedy and Justice Samuel Alito posed tough questions about whether the federal government was overreaching with the statute. Kennedy said DOMA did not seem to recognize states's "historical" responsibility for marriage and suggested the central question of the case is whether the federal government has the authority to regulate marriage.

Both attorneys arguing to strike down DOMA refused to make a federalist argument against the law, however -- instead insisting it was a discrimination case.

Before even getting to the merits of the case, the justices spent nearly an hour grappling with whether they should decide it at all because of procedural issues.They appointed Harvard professor Vicki Jackson to make the case that House Republicans do not have the legal right, or standing, to appeal the lower court's decision.

Several justices were also critical of the Obama administration's decision to stop defending the law in court while still enforcing it. Roberts appeared to have serious doubts about the case's procedural issues, repeatedly saying that it is "unprecedented" for the U.S. government to appeal a case even when the parties agreed with a lower court's ruling.

The two gay marriage cases before the court this term have been dogged by procedural concerns, as both were left orphaned by public officials who no longer wanted to defend them.

On Tuesday, Kennedy wondered whether the court should have agreed to hear the Proposition 8 case at all. Other justices suggested they were skeptical that supporters of Proposition 8 had standing to appeal the case once California officials decided to drop it.

It's possible that neither case could end with a decision. In DOMA, that means the lower court's decision would stand and DOMA would be illegal in the Third Circuit. The plaintiff, Edith Windsor, would be repaid the $360,000 she had to pay in estate taxes when her wife died because the government didn't recognize her marriage in New York, where gay marriage is legal. In the Proposition 8 case, gay marriage would most likely become legal in California if the justices throw it out on standing or do not reach a majority.