A group of more than 80 Republicans will file a "friend of the court" brief with the U.S. Supreme Court in favor of same-sex marriage.
The group includes some members of the House of Representatives, four former governors, and the daughter of former Vice President Dick Cheney.
"Conservatives should start to lead again and push their states to join the nine others that allow all their citizens to marry," former Utah governor and Republican presidential candidate Jon Huntsman, Jr. wrote.
"All Americans should be treated equally by the law, whether they marry in a church, another religious institution, or a town hall. Civil equality is compatible with, and indeed promotes, freedom of conscience," he concluded.
The High Court will hear cases involving same-sex marriage next month.