A law intended to prevent the breakup of American Indian families may be headed to the U.S. Supreme Court, and that has tribes worried.

The New Orleans-based U.S. Court of Appeals for the Fifth Circuit recently revived the Indian Child Welfare Act in a 2-1 decision. That came after a district court judge, who was seemingly handpicked to rule against the law, found it unconstitutional.

The 1978 law’s protections of “Indian children” in adoptions isn’t an unconstitutional race-based classification, but rather a political one, the Fifth Circuit said.

Tribal advocates argue that a contrary ruling—like the one the district court...