A federal appeals court appeared skeptical of President Donald Trump‘s sweeping tariff policy on Thursday, grilling the Justice Department on whether the president implemented the key part of his trade policy lawfully.

The U.S. Constitution gives the power to impose taxes — including tariffs — to Congress. But lawmakers have gradually relinquished power over trade policy to the White House. And Trump has made the most of the power vacuum, raising the average U.S. tariff to more than 18%, highest since 1934, according to the Budget Lab at Yale University.