A federal appeals court on Thursday is set to consider the validity of special counsel Robert Mueller’s appointment to investigate Russian interference in the 2016 presidential election.

The appeal, brought by an associate of Roger Stone, a longtime adviser to President Donald Trump, challenges the constitutionality of Mueller’s role in a case that could eventually reach the Supreme Court.

Andrew Miller, a former assistant to Stone, appealed after losing his bid to block a grand jury subpoena from Mueller. Miller was held in contempt, but that ruling is on hold pending the outcome at the U.S. Court of Appeals for the D.C. Circuit.

The special counsel’s team has sought to interview a number of Stone associates or have them appear before the grand jury as part of the 18-month-old probe.

Two District Court judges in Washington – one nominated by a Democrat, the other by Trump – have upheld the constitutionality of Mueller’s appointment in recent rulings. The hearing Thursday, however, is the first time an appeals court panel, made up of Judges Karen LeCraft Henderson, Judith W. Rogers and Sri Srinivasan, will review the special counsel’s authority.

“Under any possible test, Mueller clearly passes constitutional muster,” said Elizabeth Wydra, president of the liberal Constitutional Accountability Center, who submitted a brief from legal scholars.

Miller’s court challenge is backed by the National Legal and Policy Center, a conservative nonprofit, and shaped in part by arguments advanced by law professor Steven Calabresi, co-founder of the Federalist Society.

In court filings, Miller’s legal team says Mueller was named unlawfully in violation of the Appointments Clause of the Constitution because he has broad prosecutorial powers and a “lack of supervision and control over this conduct.”

Deputy Attorney General Rod Rosenstein appointed Mueller in May 2017, after Trump fired FBI Director James Comey. Rosenstein’s involvement came because then-Attorney General Jeff Sessions had recused himself from matters involving the campaign.

The president this week claimed he has the power to immediately cut short the special counsel’s investigation, but said he wants to “let it go on.”

“I could fire everybody right now,” Trump said at a post-election news conference Wednesday, shortly before Sessions resigned at the president’s request and was replaced on an acting basis with Matthew Whitaker. Whitaker, who was serving as Sessions’s chief of staff would assume authority over the special counsel probe, a Justice Department official said Wednesday.

The attorney general has the power “to move his pieces in the criminal law enforcement chess game all over the board, but it does not give him the power to create a new Queen,” according to the challenge from Miller’s team, led by attorney Paul Kamenar.

The Russian firm, Concord Management and Consulting, will make a similar argument at the appeals court Thursday in an effort to have an indictment of the company issued by the special counsel dismissed. Concord was charged along with 13 Russian nationals and two other companies, accused of orchestrating a social media influence campaign to affect the 2016 election.

The three-judge panel must answer the specific question of whether Mueller is a “principal officer,” requiring appointment by the president and Senate confirmation, or an “inferior” one, who can be appointed by the head of a department.

The clause generally requires officers to be nominated by the president and confirmed by the Senate, but allows for another group of officials deemed “inferior” to be appointed only by the heads of departments, the president or a court of law if they are supervised by a principal officer. If Mueller is a “principal” officer, his appointment is invalid because he was tapped by Rosenstein.

Mueller’s team, to be led in court Thursday by attorney Michael Dreeben, says the appointment is constitutional because the attorney general has the authority to name special counsels to “detect and prosecute crimes” and to define their duties. An officer is “inferior,” according to Mueller’s team, if he is subject to supervision by officials who are appointed by the president and confirmed by the Senate.

Under the special counsel regulation, Rosenstein has supervised Mueller’s work and could remove him from his post for misconduct and “good cause,” including violation of Justice Department policies. “The Attorney General receives a regular flow of information about the Special Counsel’s actions; he can demand an explanation for any of them; and he has power to intervene when he deems it appropriate to prevent a deviation from established Departmental practices,” according to Mueller’s team.

In a July ruling in Miller’s case, Chief U.S. District Judge Beryl Howell wrote the “scope of the Special Counsel’s power falls well within the boundaries the Constitution permits,” in part because he is supervised by an official, in this case Rosenstein, “who is himself accountable to the elected President.”

Miller worked for longtime Trump friend Stone during the 2016 presidential campaign, handling duties such as setting up media interviews. He is one of at least nine of Stone’s associates contacted by prosecutors so far.

Stone has repeatedly said he was not in touch with WikiLeaks, which published Democratic emails that prosecutors allege were hacked by Russian operatives and that were published in the final months of the presidential race.

After Howell’s ruling, an opinion in August from U.S. District Judge Dabney Friedrich, a Trump appointee, also upheld Mueller’s appointment. Friedrich rejected a separate challenge from the Russian firm, Concord, accused of funding an Internet trolling operation targeting U.S. voters. Concord has pleaded not guilty to charges it conspired to obstruct the election.