The brief is in support of the plaintiffs in the Hollingsworth v. Perry case now before the Supreme Court, which challenges California's Proposition 8 barring same-sex marriage, reports CBS News.
The case, which will be argued starting in late March, could result in the invalidation of statewide bans on same-sex marriage across the country. It is one of two same-sex marriage cases being considered this term by the Supreme Court; the other challenges the Defense of Marriage Act, which bars federal recognition of same-sex marriage.
Among the signatories to the letter are former Republican Governors Christie Todd Whitman and Bill Weld; Rep. Ileana Ros-Lehtinen, R-Fla.; former Republican Reps. Deborah Pryce and Mary Bono Mack; 2012 presidential candidates and former governors Gary Johnson and Jon Huntsman; and former Republican National Committee chair Ken Mehlman, the onetime George W. Bush campaign manager who has since come out as gay.
However, not among the signatories are some Republicans who have expressed support for same-sex marriage in the past, including Dick Cheney and Laura Bush.
The American Foundation for Equal Rights, which organized the effort, said more names will be added before the brief is filed.
CBS News polling has found that a majority of Americans believe same-sex marriage should be legal, though more than six in ten said it should be left to the states to decide.