President Trump pushed closer toward a constitutional crisis with Congress Wednesday, vowing to fight all subpoenas from House Democrats ordering current and former aides to testify on Capitol Hill. Trump’s remarks came as the White House continued to suggest the president may invoke executive privilege to prevent former White House counsel Don McGahn from testifying publicly to the House Judiciary Committee on May 21.

President Donald Trump: “Well, the subpoena is ridiculous. We have been—I have been the most transparent president and administration in the history of our country, by far. … We’re fighting all the subpoenas. Look, these aren’t like impartial people. The Democrats are trying to win 2020.”

House Oversight Committee Chair Elijah Cummings said aides who defy subpoenas could be held in contempt of Congress, warning, “These employees and their personal attorneys should think very carefully about their own legal interests rather than being swept up in the obstruction schemes of the Trump administration.” House Judiciary Chair Jerrold Nadler has suggested fining officials personally if they refuse to testify to his committee.