Spokespersons for Ros-Lehtinen and Hanna did not respond to requests for comment. The brief is not yet final and more signatories are being recruited, TPM has learned.
The effort signals a recognition among some leading Republicans that same sex marriage -- which a growing majority of Americans support -- is inevitable, and it highlights a growing schism with social conservatives, who serve as critical foot-soldiers for the GOP during elections and remain vigorously opposed to gay marriage.
The brief comes shortly after Huntsman wrote an op-ed in the American Conservative magazine describing "marriage equality" as "a conservative cause." Earlier this month, Illinois' GOP chairman Pat Brady ruffled feathers among conservatives after signaling his support for same sex marriage and declaring his party on "the wrong side of history" over the issue.
Public support is also growing among Republicans. According to Gallup, support for legal gay marriage grew from 22 to 30 percent among self-identified Republican respondents between May and December of 2012.
The expected amicus brief is a snub to House Republican leaders, who are spending millions of dollars to defend the Defense of Marriage Act, which prohibits federal recognition of same sex marriage, after the Justice Department dropped its defense of the law. President Obama considers DOMA unconstitutional and came out for gay marriage last May for the first time in his presidency.
The Supreme Court is scheduled to hear oral arguments on the Prop 8 and DOMA cases in late March, with rulings expected by the end of June.