Evoking Ronald Reagan and Barry Goldwater, a group of Western-state Republicans plans to enter the battle in favor of same-sex marriage on Tuesday, urging a federal appeals court to declare gay marriage bans in Utah and Oklahoma unconstitutional.

The most prominent of the approximately 20 signers of the brief are former Senator Alan K. Simpson of Wyoming, a longtime supporter of gay rights, and former Senator Nancy L. Kassebaum of Kansas, who said last year that she had reconsidered her former opposition to same-sex marriage. The document says that “marriage is strengthened” and “the social stability of the family unit are promoted” by allowing gay and lesbian couples to marry.

The document is a friend-of-the-court brief, being filed to the United States Court of Appeals for the 10th Circuit, in Denver. That court is hearing appeals from Utah and Oklahoma to reinstate their restrictive marriage laws.

The brief was the latest sign of widening cracks in Republican opposition to same-sex marriage, even deep in the country’s conservative heartland.