WASHINGTON (AP) - The US Supreme Court, in the second of back-to-back gay marriage cases, turns on Wednesday to a constitutional challenge to the federal law that prevents legally married gay Americans from collecting federal benefits generally available to straight married couples.
A section of the 1996 Defence of Marriage Act says federal law only recognises marriage between a man and a woman, regardless of state laws that allow same-sex marriage.
Lower federal courts have struck down the measure, and now the justices, in nearly two hours of scheduled argument, will consider whether to follow suit.
Marital status is relevant in more than 1,100 federal laws that include estate taxes, federal Social Security pension survivor benefits and health benefits for federal employees.
Lawsuits around the US have led four federal district courts and two appeals courts to strike down the law's Section 3, which defines marriage.
In 2011, the Obama administration abandoned its defence of the law but continues to enforce it.
The argument follows Tuesday's case over California's ban on same-sex marriage, a case in which the justices indicated they might avoid a major national ruling on whether America's gays and lesbians have a right to marry.
Even without a significant ruling, the court appeared headed for a resolution that would mean the resumption of gay and lesbian weddings in California.
The arguments in both cases have drawn intense interest as polls show momentum rapidly swinging towards public acceptance of same-sex marriage. Thousands of people marched outside the Supreme Court building on Tuesday, loudly supporting one side or the other.
Republicans in the House of Representatives are now defending the Defence of Marriage Act in the courts. While same-sex marriage is gaining public acceptance, religious conservatives still fervently oppose it.
Same-sex marriage is legal in nine states and the district of Washington, while 12 others recognise "civil unions" or "domestic partnerships" that grant the same benefits without full rights of marriage. The other states ban gay marriage in their constitutions.
The justices chose for their review the case of Ms Edith Windsor, 83, of New York, who sued to challenge a US$363,000 (S$451,724) federal estate tax bill after her partner of 44 years died in 2009.
Ms Windsor, who goes by Edie, married Ms Thea Spyer in 2007 in Canada after doctors told them that Ms Spyer would not live much longer. Ms Spyer suffered from multiple sclerosis for many years. Ms Spyer left everything she had to Ms Windsor.
There is no dispute that if Ms Windsor had been married to a man, her estate tax bill would have been zero.
The Defence of Marriage Act was signed into the law by former President Bill Clinton, who now says times have changed and is calling for the court to strike it down. Mr Clinton's wife, potential 2016 presidential candidate Hillary Clinton, recently came out in support of gay marriage.
On Tuesday, the justices weighed a fundamental issue: Does the Constitution require that people be allowed to marry whom they choose, regardless of either partner's gender?
The fact that the question was in front of the Supreme Court at all was startling, given that no state recognised same-sex unions before 2003 when Massachusetts' high court ruled it was unconstitutional to bar same-sex couples from marrying in the state.
But it was clear from the start of the 80-minute argument in a packed courtroom that the justices, including some liberals who seemed open to gay marriage, had doubts about whether they should even be hearing the challenge to California's voter-approved gay marriage ban.
Justice Anthony Kennedy, the potentially decisive vote on a closely divided court, suggested the justices could dismiss the case with no ruling at all.
Such an outcome would almost certainly allow gay marriages to resume in California, where the ban has been struck down by lower courts, but might have no impact elsewhere. The court is not expected to rule before late June.
There was no majority apparent for any particular outcome, and many doubts were expressed by justices about the arguments advanced by lawyers for the opponents of gay marriage in California, by the supporters and by the Obama administration, which is in favour of same-sex marriage rights.
The administration's entry into the case followed President Barack Obama's declaration of support for gay marriage during last year's presidential election campaign.
On the one hand, Justice Kennedy acknowledged that same-sex unions had only become legal recently in some states, a point stressed repeatedly by Mr Charles Cooper, the lawyer for the defenders of California's ban. Mr Cooper said the court should uphold the ban as a valid expression of the people's will and let the vigorous political debate over gay marriage continue in the states.
Yet when Mr Theodore Olson, the lawyer for two California same-sex couples, urged the court to support such marriage rights everywhere, Justice Kennedy feared such a ruling would push the court into "uncharted waters."
Mr Olson said the court similarly ventured into the unknown in 1967 when it struck down bans on interracial marriage still in effect in 16 states.
The justice also made clear he did not like the rationale of the federal appeals court that struck down California's ban, even though it cited earlier Supreme Court opinions in favour of gay rights that Justice Kennedy had written.
That appeals court ruling applied only to California, where same-sex couples briefly had the right to marry under a ruling by the state's top court before voters in November 2008 adopted Proposition 8, a constitutional amendment that defined marriage as the union of a man and a woman.
Roughly 18,000 couples were wed in the nearly five months that same-sex marriage was legal and those marriages remain valid in California.
Reflecting the high interest in the cases, the court planned to release an audio recording of Wednesday's argument shortly after it concludes, just as it did Tuesday.
The Tuesday audio can be found at: http: tinyurl.com/dxefy2a.