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 Donald Trump and Iowa Republican Sen. Chuck Grassley are sparring on social media over whether the Judiciary Committee chairman should abandon a century-old tradition that allows senators to block the advancement of judicial nominees who would serve in the senators’ home states.

Senators approve of a nominee by returning their blue slip to the Judiciary Committee Chairman. If a senator does not return their blue slip or returns it with objections, the nominee does not advance. In blue states like New York, some of Trump's judicial nominees have stalled out because Democratic senators have not returned blue slips.