The case arises from a lawsuit brought by three conservation groups, the Center for Biological Diversity, the Animal Legal Defense Fund, and Defenders of Wildlife, claiming the border wall project runs afoul of environmental laws.

The groups appealed to the Supreme Court after a San Diego District Court judge in February ruled against them.

On Monday, the high court, without comment, declined to hear the challenge, dealing a blow to the conservationists’ argument that a 1996 law is contrary to the Constitution’s separation of powers. The law gives the Department of Homeland Security unchecked ability to waive legal requirements if they prevent building border protective infrastructure.

The conservation groups said building a wall along the border “would harm plants, rare wildlife habitats, threatened coastal birds like the snowy plover and California gnatcatcher, and other species such as fairy shrimp and the Quino checkerspot butterfly.”

Congress has not yet approved the $5 billion Trump has requested to pay for the border wall, a central focus of his presidential campaign. He has threatened a government shutdown if they don’t comply.