Kennedy suggested that he viewed the law as overstepping the federal government's authority because issues of marriage and children's rights are historically decided by the states.
Justice Ruth Bader Ginsburg said that, under the law, states would have two kinds of marriage — "the full marriage and then this sort of skim milk marriage." She suggested that would violate the constitutional guarantee of equal protection.
Kermit Roosevelt, a University of Pennsylvania law professor, said there was a 70 percent chance DOMA would be struck down. That and avoiding a broad constitutional ruling on gay marriage, he said, "would be a modest victory for gay rights, which is probably what the court wants at this point."
Tribe agreed. "That will be a significant victory for same-sex marriage but is unlikely to be a landmark equality precedent," he said.
Congressional Republicans support the law, arguing that it promotes traditional marriage and helps ensure that children grow up in a nurturing environment. They also contend that DOMA provides for uniform treatment of taxpayers in the various states.
The justices barely discussed the Obama administration's argument that laws that treat gays differently should be given "heightened scrutiny," a stricter standard used to protect racial minorities and women from discrimination. Giving gays such legal protection would buttress their challenges to laws barring gay marriage across the country.
Even if the court doesn't afford gays heightened scrutiny, and instead strikes down DOMA as interfering with states' rights, that would be a victory for gay rights, said Jenny S. Martinez, a Stanford Law School professor.
"That's still a rejection of the argument that the federal government has a legitimate interest in not recognizing same-sex marriages," Martinez said.
Some gay-marriage advocates are prepared to declare victory almost regardless of what the court says when it rules.
"What's clear is that we have the momentum and we have the winning strategy, and we're going to get there whether in June or the next round," said Evan Wolfson, president of Freedom to Marry, a New York-based national gay-rights group.