An unlikely coalition grouping the Obama administration, major corporations, Republican lawmakers and Clint Eastwood has found common cause in urging the US Supreme Court to legalize gay marriage.
The nine Supreme Court justices, who rule on some of the most divisive issues in American society, will take up cases on March 26 and 27 dealing with the sensitive question of same-sex unions.
Gay marriage is banned at the federal level but has been made legal in nine US states plus the capital Washington, creating a constitutional anomaly.
In an unprecedented move, President Barack Obama's administration filed a brief last week asking the high court to overturn a federal law defining marriage as a union between only one man and one woman.
The document marked the first time a president has endorsed gay marriage rights before the Supreme Court.
According to the filing, the 1996 Defense of Marriage Act "violates the fundamental constitutional guarantee of equal protection" under the 14th Amendment to the US Constitution.
The act, it said, "denies to tens of thousands of same-sex couples who are legally married under state law an array of important federal benefits that are available to legally married opposite-sex couples."
On Thursday, the government went further, with a friend-of-the-court filing in the other case before the justices: California's ban on gay marriage.
The Obama administration again invoked the "equal protection" clause, though the filing was more narrowly focused on the most populous US state's Proposition 8 and did not seek a ruling that would apply nationwide.
The document, hailed as "historic" by the Human Rights Campaign gay advocacy group, did not explicitly endorse marriage equality throughout the 50 states.
But it "could give the court a way to advance gay marriage rights, without going the full step... of declaring that marriage should be open to all same-sex couples as a constitutional requirement," court watcher Lyle Denniston wrote on his SCOTUSblog.
The administration's brief noted that seven other states -- Delaware, Hawaii, Illinois, Nevada, New Jersey, Oregon and Rhode Island -- have measures that grant same-sex couples rights similar to those of married couples, while restricting marriage to heterosexual unions.
Those states would be affected by the California ruling.
"The exclusion of gay and lesbian couples from marriage does not substantially further any important governmental interest," Verrilli wrote.
Gay marriage opponents have seized upon the same similarities to claim there is no discrimination, saying California provides essentially the same rights and obligations of marriage to same-sex domestic partners.
The Family Research Council, a conservative Christian group promoting the traditional family unit, alleged that Obama had shown "brazen hypocrisy" in changing his position on DOMA and "federalizing" a California matter.
Nine states and the US capital Washington currently allow gay marriage. The states include Connecticut, Iowa, Maine, Maryland, Massachusetts, New Hampshire, New York, Vermont and Washington.
In a surprise move, 131 Republicans -- including aides to the campaigns of Obama's former presidential rivals Mitt Romney and John McCain -- filed a brief urging the Supreme Court to scrap California's ban on gay marriage.
Actor-director Clint Eastwood, a lifelong Republican who supported Romney at the Republican National Convention, joined former George W. Bush administration officials, lawmakers and former governors in signing the brief.
"There is no legitimate, fact-based reason for denying same-sex couples the same recognition in law that is available to opposite-sex couples," the document read.
American football players, 13 US states, human rights groups, sociologists, conservative and liberal experts were among signatories to court filings calling for legalizing gay marriage or scrapping California's ban.
And major companies like Apple, Nike, Facebook and Morgan Stanley also pitched in, saying that banning gay marriage could prevent the recruitment of the best qualified workers.