The Supreme Court signalled Wednesday it could throw out a US law that defines marriage as strictly between a man and a woman, in a second day of hearings on the thorny issue of same-sex marriage.
For nearly two hours, the nine justices who make up the nation's highest court grilled lawyers on the constitutionality of the Defense of Marriage Act (DOMA) -- with five of them appearing to lean in favor of striking it down.
The controversial 1996 law denies married gay and lesbian couples the same federal rights and benefits that heterosexual couples take for granted, from tax breaks and welfare benefits to access to a hospitalized spouse.
The plaintiff is Edith Windsor, 83, who was ordered to pay federal inheritance taxes of $363,000 following the 2009 death of Thea Spyer, her partner of more than 40 years. The couple had married in Canada in 2007.
Under DOMA, the surviving half of a heterosexual couple would not have faced the same tax burden.
In an exchange with lawyers arguing the two sides of the case, Justice Ruth Bader Ginsburg suggested DOMA represented, in her view, two kinds of wedlock -- "full marriage and skim milk marriage."
Her colleague Elena Kagan went further, saying that DOMA was "infected by animus, fear and dislike."
Justice Anthony Kennedy, a conservative whose swing vote on gay rights could decide the outcome, said he was "troubled" by how the DOMA case would impact the rights of each of the 50 states to set out their own marriage laws.
Outside the court, hundreds of marriage equality supporters chanted "Edie! Edie!" as Windsor exited the stately building with her legal team and told assembled reporters: "I think it was great. I think it went beautifully."
"I thought the justices were gentle. They were direct. They asked the right questions," said the retiree, who wore a circular diamond brooch -- a de facto engagement token from Spyer from early in their relationship -- on her lapel.
"The justices asked all the questions we expected them to ask," added Windsor's lawyer Roberta Kaplan, who declined to speculate on how the court would rule in the coming months.
President Barack Obama's administration initially opposed Windsor's bid to repeal DOMA as it progressed through the lower courts, where the law has already twice been ruled unconstitutional.
But the White House has since switched sides. Now it is calling for the law to be overturned, leaving DOMA to be defended by a group of Republican lawmakers, along with a coalition of religious and conservative groups.
Opponents of Windsor's stance include Republican lawmakers and Christian groups. One of their lawyers, Paul Clement, argued DOMA was constitutional in that it applied only to laws on the federal level.
On Tuesday, the court trod cautiously as it weighed the constitutionality of California's Proposition 8, a 2008 ballot initiative that saw a majority of voters in the nation's most populous state ban same-sex marriage.
While it will take several months for them to issue a ruling, several Supreme Court justices indicated Tuesday they would be in no hurry to make a verdict that could extend the right to same-sex marriage to the entire country.
Forty-one states currently ban or limit such marriages.
Legal experts cautioned Wednesday it can be hard to predict what the Supreme Court will do based on oral arguments, but Thomas Keck of Syracuse University in New York anticipated a "mixed decision" on the same-sex marriage question.
It could hold that Washington is "constitutionally required to recognize same-sex marriages from states that allow them," but at the same time decline to rule on whether other states must allow gay marriages, he told AFP.
"If the court does get to the merits (of DOMA), I think it will find DOMA's pervasive system of discrimination unconstitutional," added Elizabeth Wydra of the Constitutional Accountability Center, a Washington legal think tank.
Successive public opinion polls have indicated that a majority of Americans now accept the principle of same-sex marriage, including an overwhelming number of younger citizens.
In May last year, Obama became the first serving US president to publicly back gay marriage.
"I think it is time for the justices to examine this issue," Obama said Wednesday in an interview with Spanish-language television network Telemundo.
"States have defined marriage and the federal government has followed the lead of the states. And so my hope is that the court reaches these issues. And that we end up living in a country where everybody is treated fairly."