The US Supreme Court on Wednesday was hearing a second day of arguments on whether same-sex couples have the right to marry.
Attention shifted to the federal Defence of Marriage Act, which prohibits the US government from offering benefits or recognizing same-sex couples even if they are legally married under state law.
Gay and lesbian couples seeking a marriage licence face a patchwork of state-by-state laws with nine states and the District of Columbia allowing them to wed and 38 others outlawing such unions.
The Supreme Court is hearing arguments this week in two cases, which have the potential to reshape the legal landscape.
The cases involve a California law known as Proposition 8 that bans same-sex marriages there, and the Defence of Marriage Act (DOMA).
Advocates of same-sex marriage hope the court's nine justices will rule that same-sex marriages are protected under the US Constitution and overturn existing bans on gay unions.
Opponents want the court to protect the traditional definition of marriage as that between a man and a woman.
The Defence of Marriage Act case involves a woman who had to pay estate taxes after her lesbian partner died. The couple had been married in Canada.
A heterosexual couple would not have had to pay the taxes under laws that do not tax property left from one spouse to the other.
President Barack Obama's Justice Department said last year it would no longer defend Congress' 1996 law.
The president's position leaves the government in the unusual position of not arguing on behalf of an existing law at the Supreme Court, allowing a group of lawmakers to argue in favour of DOMA.
The government has also filed a brief expressing agreement with the pro-gay marriage camp in the California case.
Bill Clinton, who signed DOMA into law as president, said this month it was time to overturn the law.
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