Other gay tribal citizens say they support same-sex marriage but do not consider marriage rights a priority, pointing out that many gay Navajos suffer from drug abuse and debilitating depression.

Fixing these ills, said Jeremy Yazzie, 33, who counsels gay and transgender Navajos, is far more important. “Everyone is worried about repealing the gay marriage act,” Mr. Yazzie said. “That’s far from my work. How can we love somebody else if we can’t even love ourselves?”

Mr. Nelson and Mr. Yonnie, 29, a caseworker for the tribal welfare agency, could marry in Arizona, Utah and New Mexico, the states that border the reservation, if they wanted. “These states surrounding the Navajo Nation are taking big steps forward — steps for equality,” Mr. Nelson said. “The Navajo Nation is not.”

Mr. Nelson grew up in a mobile home on the reservation, tending sheep and doing homework by kerosene lamp. He came out in an email to his family in his early 20s, was largely accepted and soon became a powerful force in reservation politics, working for two tribal presidents. He met Mr. Yonnie through Facebook about four years ago and they now live with Mr. Yonnie’s mother in this rural outpost.

“Marriage to me is security,” Mr. Yonnie said over dinner recently.

But Mr. Nelson’s efforts to sway legislators have been hindered by his damaged credibility. In 2011, he admitted to filing false claims to the police, saying he had been threatened because of his race and sexuality. At the time, Mr. Nelson said in a recent interview, he had been suffering from depression and stress after the death of a family member. “I did some stupid stuff and said some stupid things,” he said.

But he has pressed on. In the last year or so, Mr. Nelson has attended five tribal meetings to argue for changes to the law; he has briefed tribal presidential candidates on the issue and attended community events to hand out information and ask people to sign cards pledging their support. Sometimes he is accompanied by other Navajos, but other times, he and Mr. Yonnie are alone. “In many ways, it’s just been Brennen and I,” Mr. Nelson said.