A majority of the Supreme Court may be willing to strike down the controversial 1996 federal law that defines marriage as the union of a man and a woman, but perhaps not on the grounds being sought by the Obama administration and supporters of gay marriage.

That was the dominant conclusion Wednesday as the court heard arguments for the second straight day in a potentially landmark gay marriage case. On Tuesday, the justices explored the constitutionality of a 2008 California ballot initiative that banned same-sex marriage, a case that could lead to the measure being struck down and, potentially, to the nationwide legalization of such unions.

On Wednesday, Justice Anthony M. Kennedy, who is widely seen as the swing vote in the California challenge, indicated that he could provide the decisive vote in striking down the federal Defense of Marriage Act as well. Kennedy suggested strongly that the measure may infringe on the traditional right of states to regulate marriage.

The 17-year-old law, which was passed by a Republican Congress and signed by President Bill Clinton, denies more than 1,000 federal benefits to gay couples, even if those couples are married legally under state law.

By unequivocally defining marriage as the union of a man and a woman, regardless of how states choose to define their residents’ unions, the federal law is “not consistent” with the historic role of states in regulating marriage, Kennedy said.

The measure is “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,” he said.

Kennedy’s position could be a cause of celebration for gay marriage advocates, including more than 200 Democratic members of Congress who had urged the court to strike down the law. But it was clearly not the legal interpretation favored by the Obama administration’s top lawyer, Solicitor General Donald B. Verrilli Jr., who urged the court to accept a broader rationale for striking down the law: that it violates fundamental equal-protection guarantees set out in the Constitution.

Verrilli sought to persuade the justices that DOMA was not an incursion by Congress into the domain of the states but rather an act of discrimination on the part of Congress. Citing the House report accompanying the legislation, he said the law amounted to an “expression of moral disapproval” about gay marriage on the part of lawmakers.

“I think it’s time for the court to recognize that this discrimination, excluding lawfully married gay and lesbian couples from federal benefits, cannot be reconciled with our fundamental commitment to equal treatment under law,” Verrilli said. “This is discrimination in its most very basic aspect.”

Ideological Splits

The nearly two-hour argument on Wednesday, which followed an 80-minute debate on Tuesday, revealed the traditional ideological rift on the high court between its liberal and conservative justices.

Liberals generally seemed inclined to strike down DOMA on either federalism or equal protection grounds, with Justice Ruth Bader Ginsburg arguing that the law “affects every area of life” for same-sex couples and draws distinctions between what she ironically called “full marriage” and “this sort of skim-milk marriage.”

But conservatives, including Chief Justice John G. Roberts Jr., doubted Verrilli’s contention that the law represents an intentionally discriminatory act on the part of Congress.

On two occasions, Roberts questioned whether 84 senators really acted with “animus” in voting for a measure that withheld benefits from gay couples, prompting a lawyer for the woman who brought the case, Edith Windsor, to argue that the lawmakers’ votes may have been a sign of the times.

“I think what is true, Mr. Chief Justice, is that times can blind and that, back in 1996, people did not have the understanding [about gay marriage] that they have today,” said the lawyer, Roberta A. Kaplan.

Windsor, an 83-year-old woman from New York, sued to challenge a $363,000 federal estate-tax bill after her female spouse died in 2009. Had Windsor been married to a man, she would have paid no estate tax. The lower court whose opinion the justices are evaluating, the New York-based 2nd Circuit Court of Appeals, found that the Defense of Marriage Act is unconstitutional.

During Wednesday’s arguments, Roberts told Kaplan that politicians today appear to be on Windsor’s side rather than the other way around.

“As far as I can tell, political figures are falling over themselves to endorse your side of the case,” he said, possibly referring to a string of mainly Democratic senators who announced for the first time this week that they support same-sex marriage rights.

Support for Law

Paul D. Clement, the lawyer who argued in favor of the 1996 act, said it is not discriminatory and instead represents an effort by Congress to provide uniformity in federal law. After the Obama administration abandoned a legal defense of the marriage law in 2011, the House subsequently decided to take up the effort, and Republican leaders have authorized $3 million to pay Clement’s law firm.

Clement said the federal government took a cautious, rather than an activist, approach in passing the law because it sought to ensure that federal benefits would be applied equally across the country. He noted that Hawaii legalized same-sex unions and that members of Congress wanted to ensure that Hawaii would not dictate national policy.

By passing DOMA, Congress balanced its historic interest in deferring to the states on marriage with its historic interest in providing uniformity of federal laws, Clement argued.

Clement also noted that Congress did not act with “animus” because it previously asked the Justice Department for its opinion on whether DOMA is constitutional and had received assurances on three occasions that it was. Such behavior does not reflect a body that has been acting with “animus,” he said.

The first 50 minutes of Wednesday’s arguments focused on a far narrower but potentially significant legal question: whether it is permissible for the House to take up the defense of the law in the administration’s absence.

Justices on both the left and right appear to have serious concerns on that point, even though both the administration and House Republicans want the case to be considered on the merits.

At one point early in the arguments, Roberts needled the administration for asking the Supreme Court to decide the case, even though it has already won in the 2nd Circuit Court of Appeals. Apparently referring to President Barack Obama, Roberts wondered why the president doesn’t have the “courage of his convictions” to rely on the lower court ruling.