WASHINGTON – The Supreme Court refused Monday to consider a challenge to President Donald Trump's controversial appointment of Matthew Whitaker as acting attorney general.

The justices were asked in November to decide if Whitaker was legally installed as the temporary successor to ousted Attorney General Jeff Sessions. If not, that would have left Deputy Attorney General Rod Rosenstein overseeing – and presumably protecting – special counsel Robert Mueller.

Mueller's probe into Russian meddling in the 2016 election and potential coordination with the Trump campaign appears to be nearing completion. Meanwhile, attorney general nominee William Barr faces a Senate confirmation hearing this week, and Rosenstein is prepared to step down if Barr is installed.

Maryland Attorney General Brian Frosh has filed a separate challenge to Whitaker's appointment in federal district court.

Whitaker's appointment drew a wave of criticism from Democrats and some Republicans. They said he lacked authority to serve as the nation’s chief law enforcement officer because he was not confirmed by the Senate. In addition, Whitaker's past public criticism of the Russia investigation prompted Democrats to call for his recusal from overseeing the Mueller inquiry.

The Justice Department argued that Whitaker’s standing as a senior department executive authorized Trump to elevate him to attorney general. It cited the Vacancies Reform Act, which allows for the appointment of a senior staffer who has been in office for at least 90 days. Prior to his appointment, Whitaker served as Sessions' chief of staff.

But appellate lawyer Thomas Goldstein argued that an existing line of succession within the department, known as the Attorney General Succession Act, provides for the deputy attorney general or another Senate-confirmed official to serve as acting attorney general in the event of a vacancy.