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).

In the Oval Office on Monday, Trump derided the century-old practice of “blue slips,” which allows senators to effectively block prosecutor or district court nominees that a president selects from their home state. He suggested it was “unconstitutional” and said the Justice Department would pursue legal action to end the custom.