WASHINGTON — Dozens of prominent Republicans — including top advisers to former President George W. Bush, four former governors and two members of Congress — have signed a legal brief arguing that gay people have a constitutional right to marry, a position that amounts to a direct challenge to Speaker John A. Boehner and reflects the civil war in the party since the November election.

The document will be submitted this week to the Supreme Court in support of a suit seeking to strike down Proposition 8, a California ballot initiative barring same-sex marriage, and all similar bans. The court will hear back-to-back arguments next month in that case and another pivotal gay rights case that challenges the 1996 federal Defense of Marriage Act.

The Proposition 8 case already has a powerful conservative supporter: Theodore B. Olson, the former solicitor general under Mr. Bush and one of the suit’s two lead lawyers. The amicus, or friend-of-the-court, brief is being filed with Mr. Olson’s blessing. It argues, as he does, that same-sex marriage promotes family values by allowing children of gay couples to grow up in two-parent homes, and that it advances conservative values of “limited government and maximizing individual freedom.”

Legal analysts said the brief had the potential to sway conservative justices as much for the prominent names attached to it as for its legal arguments. The list of signers includes a string of Republican officials and influential thinkers — 75 as of Monday evening — who are not ordinarily associated with gay rights advocacy, including some who are speaking out for the first time and others who have changed their previous positions.