WASHINGTON — Opponents of Matt Whitaker's appointment as acting attorney general opened a new legal front Friday in the fight over his selection, asking the U.S. Supreme Court to rule that he is not legally qualified for the job.

President Donald Trump appointed him on Nov. 7, shortly after Attorney General Jeff Sessions announced that he was stepping down at the president's request. Whitaker had been serving as chief of staff to Sessions.

The latest move to contest the appointment came in a lawsuit pending before the Supreme Court against the U.S. attorney general, who was Jeff Sessions when the case was filed in June. The challengers asked the justices to rule that the name on the case should now be Rod Rosenstein, who they said is actually the acting attorney general.

Whitaker cannot be in the position, they contend, because he wasn't confirmed by the Senate. Their Supreme Court filing said the Constitution requires confirmation for a cabinet-level job, even an acting one. And a separate federal law provides that the deputy attorney general automatically takes over when the attorney general's office is vacant, they told the justices.

Tom Goldstein, a Washington lawyer who filed the motion Friday, said this appears to be the first time such a request has been made, asking the justices to rule on the identity of the correct defendant before the court has decided whether to hear the case at all.

"This is the extraordinary case in which the identity of the successor is both contested and has important implications for the administration of justice nationally," he said.

Goldstein was the lawyer behind a similar challenge to Whitaker's appointment that was filed Tuesday in federal court by the state of Maryland.

The Supreme Court case was originally brought by a Nevada man, Barry Michaels, who asked the justices to rule that the right to own a gun should not be taken away from someone convicted of certain non-violent felonies.

The Trump administration vigorously defends Whitaker's appointment. An opinion from the Justice Department's Office of Legal Counsel said the government has identified over 160 times throughout U.S. history when presidents appointed non-Senate confirmed government officials to serve in high level positions. In recent years, the opinion said, such appointments were made by Presidents George W. Bush and Barack Obama.

The opinion said while courts have held that the Constitution requires principal officers to be confirmed by the Senate, "it does not follow that Acting Attorney General should be understood to be one," because the person appointed to serve in that capacity is not in a "continuing and permanent" position.