Nippon Steel, whose multibillion-dollar proposal to buy struggling US Steel was blocked by President Joe Biden last week, says it is considering taking legal action against the US government.

The companies argued in a lawsuit announced on Monday that Biden violated the Constitution by blocking the merger via a sham national security review that deprived the companies of their right to a fair process.

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.