President Donald Trump threatened legal action over a Senate tradition Democrats have used to block certain judicial nominees, escalating a long-running dispute with Senate Judiciary Committee Chairman Chuck Grassley (R-IA).

President Trump is escalating his criticism of Senate Judiciary Committee Chair Chuck Grassley (R-Iowa) for his unwillingness to overhaul the “blue slip,” a move that has further dismayed Senate Republicans.

The Judiciary Committee chair, and his fellow Republicans, have so far brushed off pressure from the president to nix the long-standing practice.