The US Supreme Court tackled same-sex unions for a second day Wednesday, indicating it might throw out a federal law that defines marriage in strictly one-man, one-woman terms.
For nearly two hours, its nine justices questioned 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 prevents couples who have tied the knot in nine states -- where same-sex marriage is legal -- from enjoying the same federal rights and benefits as heterosexual couples.
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.
The surviving half of a heterosexual couple would not have faced the same tax demand. Windsor is challenging Section 3 of DOMA on the grounds it is discriminatory because it defines marriage as between a man and a woman.
Justice Ruth Ginsburg suggested DOMA represented two kinds of wedlock -- "full marriage and skim milk marriage" -- while her colleague Elena Kagan said DOMA was "infected by animus, fear and dislike."
Justice Anthony Kennedy, a conservative whose swing vote on gay and lesbian rights could decide the outcome, said he was "troubled" by how the DOMA case impacted on the rights of states to set out their own marriage laws.
After the morning hearing, hundreds of supporters chanted "Edie! Edie!" as Windsor exited the Supreme Court with her legal team and told 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 broach -- 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 they would rule in the coming months.
President Barack Obama's administration initially opposed Windsor's bid to repeal Section 3 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.
"The case is pretty simple. It's about discrimination," said James Esseks, one of Windsor's lawyers.
"It doesn't make sense in America for a federal government to treat two different people, married under the same state law, different ways. That is unfair, it is un-American and it should be unconstitutional."
Opponents of Windsor's case dispute the portrayal of homosexuals as the victims of discrimination, describing them in a brief as "one of the most influential, best-connected, and best-organized groups in modern politics."
On Tuesday, the court trod cautiously as it considered the constitutionality of California's Proposition 8, a 2008 ballot initiative that saw a majority of voters ban same-sex marriage in the nation's most populous state.
While it will take months 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.
Kennedy, whose remarks are closely watched as he is often the swing vote on the bench, voiced reluctance about the court stepping into "uncharted waters" on a case involving Californian law.
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.