A federal appeals court Thursday heard a case brought by a former aide to Trump adviser Roger Stone Roger Jason StoneThe agony of justice Our Constitution is under attack by Attorney General William Barr Justice IG investigating Stone sentencing: report MORE seeking to undermine special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s authority to continue his probe .

Andrew Miller issued an appeal in front of the three-judge panel for the DC Appeals Court to argue that Mueller’s appointment by Deputy Attorney General Rod Rosenstein Rod RosensteinDOJ kept investigators from completing probe of Trump ties to Russia: report Five takeaways from final Senate Intel Russia report FBI officials hid copies of Russia probe documents fearing Trump interference: book MORE last year was unconstitutional, according to POLITICO.

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Mueller’s authority was again thrust into the spotlight Wednesday after President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE dismissed Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE and installed a loyalist who had criticized Mueller in the past as his temporary replacement.

Judge Karen Henderson told lawyers on each side to look past Sessions’ resignation, saying, “Argue this case as if it was being argued yesterday morning.”

Miller’s lawyer, Paul Kamenar, focused much of his case on Mueller’s extensive powers, saying the regulations under which he was appointed give him “free reign.”

“The special counsel does exercise extraordinary prosecutorial and governmental powers,” POLITICO reports Kamenar told the judges. “He can bring indictments in multiple jurisdictions. He’s like a U.S. attorney at large.”

“He can indict without consulting the acting attorney general,” he added.

Deputy Solicitor General Michael Dreeben, who is representing the special counsel’s office, dismissed Kamenar’s claims.

“I think the court may have been given a slightly different impression than the way the regulations actually operate,” Dreeben said. “We are therefore required to submit reports to the acting attorney general in accordance with Department of Justice urgent report guidelines. ... Major events in investigations are reported up the chain of command.”

“The acting attorney general can ask the special counsel for explanation of any investigatory step,” he added. “He’s aware of what we’re doing. ... It’s not the case that the special counsel is off wandering in a free-floating environment and can decide when to report.”

Miller’s attorneys have suggested the case will go all the way to the Supreme Court, with Kamenar and Miller both noting that the new arrival of Justice Brett Kavanaugh Brett Michael KavanaughOvernight Health Care: US coronavirus deaths hit 200,000 | Ginsburg's death puts future of ObamaCare at risk | Federal panel delays vote on initial COVID-19 vaccine distribution Senate GOP sees early Supreme Court vote as political booster shot Trump says he'll make Supreme Court pick on Saturday MORE.

“He’d be a great justice to rule on this case,” Kamenar said, noting the justice has written about the constitutionality of the prior independent counsel law.

The special counsel’s interest in Miller arises from Mueller’s line of inquiry into Stone, who is suspected of linking up with Wikileaks, seen by many as a tool of the Kremlin, during the campaign to coordinate its leaking of emails and documents embarrassing to Democrats.

Miller’s stance against Mueller’s investigation is supported by the president, who has called the probe illegal and a “witch hunt.”

Mueller is investigating into obstruction of justice and possible collusion between the Trump campaign and Russia.