Around 75 leading Republican figures have signed a briefing urging the Supreme Court to strike down Proposition 8, in a move which could have a significant effect on the Republican Party and equal marriage in the US.
A friend of the court brief has been filed to the US Supreme Court by the Republicans.
The Supreme Court is due on 26 March to take up the case of whether to overturn Proposition 8, which in 2008 added a clause to the Californian constitution stating that marriage could only be recognised by the state if it were between a man and a woman, causing widespread controversy.
The New York Times reports that as of Monday, 75 Republicans had added their signatures to an amicus curiae briefing urging the Supreme Court to strike down Proposition 8.
The briefing follows the model of one filed earlier this week by leading lawyer and former solicitor general Ted Olson, who argues that Proposition 8 damages family values by denying the dignity of the thousands of American families with same-sex parents.
Several high-profile conservatives supporters of equal marriage are missing from the list: Dick Cheney and Colin Powell have not signed, nor has Laura Bush, who last week requested to be removed from a pro-same-sex marriage advert.
The list does include a crowd of high-profile Republicans, including some who had previously opposed equal marriage, such as Meg Whitman, who ran for the Governor of California pledging to uphold Proposition 8, and former Congresswoman Deborah Pryce.
Ms Pryce said: “Like a lot of the country, my views have evolved on this from the first day I set foot in Congress. I think [equal marriage] is just the right thing, and I think it’s on solid legal footing, too.”
Former Utah governor and potential 2016 presidential candidate Jon Huntsman appears on the list.
In it he said: “I’ve been married for 29 years. My marriage has been the greatest joy of my life. There is nothing conservative about denying other Americans the ability to forge that same relationship with the person they love.”
Among the other 70-odd Republicans on the list are Representatives Ileana Ros-Lehtinen and Richard Hanna, and members of the Bush and Reagan Presidential administrations.
Steve Schmidt, a senior advisor to 2008 Republican presidential candidate John McCain, said he signed the briefing as the future of marriage equality was a foregone conclusion given attitudes to marriage equality among young people.
“The die is cast on this issue when you look at the percentage of younger voters who support gay marriage,” said Mr Schmidt.
Analysts have said that the Republican backing could be more persuasive than the briefings filed by LGBT advocates or even the brief filed by the Obama administration on Friday, as it may appeal to conservative judges.
“If you’re trying to persuade someone like that, you can’t persuade them from the perspective of gay rights advocacy,” said Scoutusblog publisher Tim Goldstein, who believes that the Republican brief may appeal to the kind of Supreme Court judge “who respects traditional marriage but nonetheless is sympathetic to the claims that this is just another form of hatred.”
The move could put the signatories at odds with the majority of their fellow Party members in the House of Representatives, who recently upped their budget for defending the anti-marriage equality Defence of Marriage Act to $3 million (£2 million).