Office of the Solicitor General at the U.S. Department of Justice in Washington, D.C. Credit: Mike Scarcella/ ALM

The U.S. Supreme Court should not disturb a federal appeals court's appointment of a special prosecutor who will defend the criminal contempt order lodged against President Donald Trump ally and former Arizona Sheriff Joe Arpaio, the Justice Department said in a new court filing.

The special prosecutor, Christopher Caldwell of Boies Schiller Flexner, was appointed by the U.S. Court of Appeals for the Ninth Circuit last year. The appointment came after the Justice Department announced it would back Arpaio's push to vacate his contempt conviction after he received a pardon from Trump.

Trump's pardon of Arpaio, although not squarely challenged in the Ninth Circuit and Supreme Court cases, looms large in the backdrop of the court action. The pardon drew a surge of criticism by some lawyers, who argued the president's use of executive power benefited a political supporter and interfered with the proper functioning of the courts.

Arpaio's case tees up a rare fight over the practical consequences of a presidential pardon and the power of courts to appoint special prosecutors to represent specific interests. The Justice Department has resisted any push to challenge the lawfulness of Arpaio's pardon.

Noel Francisco, the U.S. solicitor general, told the justices Wednesday there is no need—at this point—to overturn Caldwell’s appointment as the special prosecutor.

"The special-prosecutor appointment can and should be narrowly construed as tantamount to the appointment of an amicus curiae," Francisco wrote. He added: "So construed, the order would avoid the serious constitutional concerns posed by a more broadly empowered special prosecutor."

Caldwell's appointment concerned the Justice Department, to be sure, but government lawyers decided not to appeal the order to the Supreme Court.

Arpaio's lawyers, including Dennis Wilenchik of Wilenchik & Bartness, want the justices to block the special prosecutor from having any role in the case as it unfolds in the Ninth Circuit. They contend the appointment of Caldwell violates separation of powers and that only the Justice Department can speak for the United States.

"The lower court has no power to replace the Department of Justice as prosecutors for the United States in a criminal case—including, if not particularly in, a prosecution for contempt of court," Wilenchik wrote in his petition at the Supreme Court.

Caldwell, for his part, told the Supreme Court on Wednesday it should turn down Arpaio's effort challenging his appointment.

Arpaio "does not come close to carrying the heavy burden that he bears in seeking such extraordinary relief," said Caldwell, a partner in the Los Angeles office of Boies Schiller who focuses on the entertainment industry, white-collar defense, securities and professional liability.

Francisco's brief raised the possibility that the government would return to the Supreme Court in the event Caldwell's role, as it plays out in the Ninth Circuit, was more than a friend-of-the-court.