WASHINGTON -- The federal Defense of Marriage Act may be hanging by a thread after a Supreme Court oral argument Wednesday that exposed sharp divisions over the 1996 law prohibiting same-sex married couples from obtaining myriad federal benefits.

A day after confronting California’s Proposition 8, which bans same-sex marriage, justices showed similar splits over whether the federal government may deny benefits to couples married in the nine states that recognize gay marriage. The division during an unusually long oral argument largely fell along traditional lines, with Justice Anthony Kennedy once again holding the middle ground.

“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,” Kennedy told the attorney who was defending the federal law.

Kennedy’s questions and asides during the 115-minute-long argument hinted, but by no means proved, that he might join the court’s liberal wing in striking down the portion of the Defense of Marriage Act that prohibits same-sex couples from receiving federal benefits. Other portions of the law, including one that allows states not to recognize gay marriages performed in other states, aren’t being challenged.

Justice Elena Kagan, the newest justice appointed by President Barack Obama, clearly articulated the liberal position, quoting from a House of Representatives report explaining that the 1996 law was meant to convey “moral disapproval of homosexuality.” One of the law’s chief backers, Sen. Tom Coburn, R-Okla., elaborated during House debate at the time that homosexual conduct was “based on perversion and . . . based on lust.”

“When Congress targets a group that is not everybody’s favorite group in the world . . . do we think that Congress’ judgment was infected by dislike, by fear, by animus and so forth?” Kagan asked the attorney who was supporting the law.

She said there was “a pretty good red flag that that’s what was going on.”

From the other side, Chief Justice John Roberts Jr. joined Justice Antonin Scalia in sounding sympathetic to the law, which defines marriage so that more than 1,100 federal benefits flow only to traditional, opposite-sex couples.

Roberts expressed doubt that Congress “had in mind same-sex couples” when it wrote “every one of those provisions.”

The case, called United States v. Windsor, posed two basic questions:

The first, debated for about 50 minutes, was whether House Republicans have the legal authority to defend the 1996 federal law now that the Obama administration has abandoned it. The second, hashed over for about 60 minutes, was whether the law violates the equal protection guarantees of the Constitution.

As with the California case, justices first puzzled over whether they should even be considering the Defense of Marriage Act case. Because the Obama administration no longer defends the law in court, House Republican leaders have funded its appellate defense. Tough questioning Wednesday brought out possible problems with this approach, including the fact that neither the Senate nor House Democrats agreed to join the House Republicans in defending the law.

“From where do they derive the right, the statutory right, to take on that responsibility of representing the House in items outside of the House?” Justice Sonia Sotomayor asked.