U.S. Steel and Japan’s Nippon Steel sued the United States government on Monday in a last-ditch attempt to revive their attempted merger after President Biden blocked it last week on the basis that the transaction posed a threat to national security.

Nippon Steel Chief Executive Eiji Hashimoto says the top Japanese steelmaker is standing firm on its proposed $15 billion acquisition of U.S. Steel

The first lawsuit filed in the U.S. Court of Appeals for the District of Columbia Circuit claims the companies’ constitutional right to due process was violated.