President Trump asked the Supreme Court on Thursday to stop a prosecutor’s investigation of his personal finances, a bold assertion of presidential power that seeks a landmark decision from the nation’s highest court.

The filing by the president’s private lawyers represents a historical moment that tests the court’s independence and highlights the Constitution’s separation-of-powers design. It also marks a new phase in the investigations that have dogged Trump throughout his presidency and have culminated in an impeachment inquiry.

The case involves Manhattan District Attorney Cyrus Vance Jr.’s attempt to enforce a grand jury subpoena issued to the president’s accountants for eight years of Trump’s tax records.

Trump went to court to block the subpoena, making a broad claim that presidents are immune from investigation while in office. A district judge and a panel of the U.S. Court of Appeals for the 2nd Circuit ruled against him, saying the subpoena was proper and the president’s longtime accounting firm, Mazars USA, must comply.

“For the first time in our nation’s history, a state or local prosecutor has launched a criminal investigation of the President of the United States and subjected him to coercive criminal process,” wrote Jay Alan Sekulow, the president’s lawyer.