More than two dozen Republicans — including a top adviser to Mitt Romney, the 2012 Republican presidential nominee, and a former congresswoman who made banning same-sex marriage her signature issue — have added their names to a legal brief urging the Supreme Court to declare that gay couples have a constitutional right to wed.

The brief comes as the White House is considering whether to weigh in on the same-sex marriage case; at this point, the Republicans who signed the document are taking a more expansive stance than [Alleged] President Obama, who favors same-sex marriage but has said he would leave it to the states, as opposed to making it a constitutional right.

The list of Republicans on the brief now tallies more 100, organizers say. It now includes Beth Myers, who ran Mr. Romney’s 2008 campaign and was a senior adviser to him in 2012, and Marilyn Musgrave, a retired Colorado congresswoman who was once rated the most conservative member of the House by the American Conservative Union.

Ms. Musgrave, who lost her bid for a fourth term in 2008, was an unsuccessful sponsor of a constitutional amendment to bar same-sex couples from marrying; she once warned that if gay couples were allowed to wed, “the next step is polygamy or group marriage.”

The brief, organized by Ken Mehlman, a former chairman of the Republican National Committee who is gay, will be filed on Thursday as a friend-of-the-court, or amicus, brief to a lawsuit that seeks to overturn Proposition 8, a California ballot initiative that bars same-sex marriage, and all similar bans.