The tribes have said that they are political entities, not racial groups. Doing away with that distinction, which underpins tribal rights, they argued, could imperil nearly every aspect of Indian law and policy, including measures that govern access to land, water and gambling.

The majority dismissed the equal protection argument, saying that no party in the case had legal standing. Instead, the justices focused on Congress’s longstanding authority to make laws about tribes and rejected claims by the challengers in the case, Jennifer and Chad Brackeen, a Christian couple from Texas, that states, not the federal government, should be addressing issues of family law.

“Our cases leave little doubt that Congress’s power in this field is muscular, superseding both tribal and state authority,” Justice Barrett wrote, adding that its authority touched on subjects as varied as criminal defense, domestic violence, property law, employment and trade. She added, “The Constitution does not erect a firewall around family law.”

In their dissenting opinions, Justices Thomas and Alito asserted that Congress had overstepped.

The majority, Justice Alito wrote, had lost sight of those most at risk: children.

The majority “decides one question after another in a way that disserves the rights and interests of these children and their parents, as well as our Constitution’s division of federal and state authority,” he added.