WASHINGTONWASHINGTON (Reuters) - The U.S. Supreme Court on Monday stayed out of the fight over whether President Donald Trump's appointment of Matthew Whitaker as acting attorney general is unlawful by rejecting a motion relating to the matter filed in a pending firearms-related case.

The court turned away the request made by Barry Michaels, a criminal defendant in a federal case whose lawyers challenged Whitaker, a former federal prosecutor, being named in court papers as the acting attorney general after Trump fired Attorney General Jeff Sessions on Nov. 7. The court in a brief order also declined to hear Michaels' appeal in the underlying case.

Michaels filed suit in Nevada challenging a U.S. law that bars him from buying a firearm due to prior non-violent criminal convictions. His lawyers decided to make Whitaker's appointment an issue in their pending appeal before the high court because Sessions was originally named as a defendant in the case.

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Michaels' lawyers argued that Deputy Attorney General Rod Rosenstein, the department's No. 2 official, should have succeeded Sessions under a federal law that vests full authority in the deputy attorney general should the office of attorney general become vacant.

Trump has nominated William Barr, who served as attorney general under former President George H.W. Bush, to succeed Sessions as attorney general. The Senate is due to begin its confirmation hearing on Barr's nomination on Tuesday. Rosenstein is preparing to leave his job soon after Barr takes office, a department official said last week.

Some of the same lawyers are involved in a similar effort to challenge Whitaker's appointment brought before a federal judge in Maryland.

Maryland Attorney General Brian Frosh asked a federal judge to bar Whitaker from appearing in an official capacity as acting attorney general in the state's lawsuit against the Trump administration over the Obamacare healthcare law.

Maryland also argued Trump violated the Constitution's "appointments clause" because the attorney general is a "principal officer" who must be appointed by the president and confirmed by the Senate.

A similar lawsuit has been filed by Democratic U.S. senators.

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The Justice Department has defended the legality of Whitaker's appointment, saying Trump was empowered to give him the job under a 1998 law called the Federal Vacancies Reform Act even though he was not a Senate-confirmed official.

Congressional Democrats had raised concerns Whitaker could undermine Special Counsel Robert Mueller's investigation into possible conspiracy between Trump's 2016 presidential campaign and Moscow.