Born exposed to drugs in 2013, an Arizona girl was immediately placed into foster care. Sixteen months later, she is living with the same foster parents, who desperately want to adopt the child.

But they can’t. The girl is American Indian, and her foster parents are not. The girl’s tribe, the Gila River Indian Community in Arizona, is fighting the adoption, citing a 1978 federal law that seeks to keep Indian families together.

Now an advocacy group has joined with the girl’s foster parents to mount the most significant challenge to the Indian Child Welfare Act since its passage, saying it creates a separate, unequal class for Indian children and is therefore unconstitutional.

If the suit is successful, the number of Indian children removed from their homes in Minnesota could dramatically increase in a state that already has the highest disparity in the country, according to federal data. Though Indian children account for only 2 percent of the state’s child population in 2014, they accounted for 24 percent of the foster care population.

ICWA was meant to address what was described at the time in Congress as “the most tragic and destructive aspect of American Indian life.” For decades, tribes across the country were ravaged when Indian children were wrongfully pulled from their homes and put into mostly white foster families.

Under ICWA, social workers must work harder to avoid removing Indian children from their homes than they would for non-Indian children. If foster care and adoption are necessary, then active efforts need to be made to place the child with an Indian family.

The Goldwater Institute, the Arizona-based conservative advocacy group that is bringing the suit, says those requirements are discriminatory because they apply only to Indian children, even those who have no connection to their tribes.

Goldwater Institute also argues that ICWA harms Indian children by making it more difficult for them to be removed from abusive or neglectful homes. And the limited supply of available Indian adoptive families means it can take longer to find a child a permanent family.

“ICWA was enacted for good intentions,” said Adi Dynar, a research attorney at the institute. “But the way it’s been applied throughout the U.S. has been very bad in taking care of the best interests of children.”

Several Indian groups across the country counter that the lawsuit would “undo decades of progress in child welfare made possible by ICWA.”

America’s history of assimilating Indian children into white families dates to the 1860s, according to a legal brief filed by the National Congress of American Indians and other groups. A superintendent of Indian Affairs said in 1865 that the country’s policy should be that “Indian children over five years old should be taken away from under the authority and influence of their savage parents (from whom they absorb only poisonous barbarism) and placed wholly under the control of white male and female teachers.” Thousands of Indian children under that policy were sent to white boarding schools to be assimilated.

When boarding schools went out of favor in the mid-20th century, states implemented a federal policy that mandated Indian children be adopted primarily to non-Indian families “in order to reduce the populations of Indian reservations, lower the federal government’s cost obligations at boarding schools, and satisfy a growing demand for adoptive children.”

By the 1970s, when Congress passed ICWA, “between 25 and 35 percent of all Indian children nationwide had been removed from their families with about 90 percent of those children placed in non-Indian homes.”

The National Congress of American Indians and other pro-ICWA groups have filed a motion seeking to have Goldwater’s suit dismissed. Goldwater is seeking approval for a class-action suit that would add all Arizona Indian children and non-Indian foster families as plaintiffs.