A federal appeals court has blocked above-ground construction of President Donald Trump’s proposed $400 million White House ballroom, preserving a preliminary injunction that prevents the project from moving forward while the administration seeks further review. The decision puts a high-profile spotlight on a basic constitutional question: whether the executive branch can unilaterally authorize a major structural change to the White House complex, or whether Congress must expressly approve it.

The dispute, brought by the National Trust for Historic Preservation, has quickly become more than a fight over one building project.