The unsigned majority opinion rejected arguments from Flynn and the Justice Department that Sullivan gravely intruded on executive branch authority by allowing the planned hearing and inviting a presentation from a retired judge whom Sullivan appointed to argue against the government’s motion to dismiss the case.

“The only separation-of-powers question we must answer at this juncture is whether the appointment of an amicus and the scheduling of briefing and argument is a clearly, indisputably impermissible intrusion upon Executive authority, because that is all that the District Judge has ordered at this point,” the majority opinion said. “We have no trouble answering that question in the negative, because precedent and experience have recognized the authority of courts to appoint an amicus to assist their decision-making in similar circumstances.

Acting through a government-paid private lawyer, Sullivan took the unusual step of seeking a full-bench rehearing of the initial appeals court decision. In the new round of the litigation, Flynn picked up no votes. The only judges to side with him Monday were the two Republican appointees who ruled in his favor in June: George W. Bush appointee Karen Henderson and Trump appointee Neomi Rao, each of whom filed a dissent.

The case appeared to produce acrimony on the powerful, Washington-based appeals court, as judges on each side accused those on the other of unprincipled reversals in position.

The majority opinion noted that the dissenters initially rejected Flynn’s request to have Sullivan’s case reassigned to another judge, but in Monday’s ruling said such a reassignment was merited.

“It is the trial judge’s conduct since the government’s May 2020 motion to dismiss, weighed in light of his earlier conduct, that delivers the coup de grâce to the last shred of the trial judge’s appearance of impartiality,” Henderson wrote.

Borrowing a vivid phrase from Henry David Thoreau, she went on to describe Sullivan’s decision to seek en banc review of the initial appeals court ruling “the trout in the milk.” Although she voted against removing Sullivan from the case just a couple of months ago, in Monday’s ruling she accused him of “glaring partiality.”

And the dissenters noted that after agreeing to rehear the case at Sullivan’s request, the majority sidestepped the validity of that move by declaring that the broader court decided at its own initiative to take a fresh look at the dispute.

Rao complained that move was “inconsistent with this court’s established practice.”

Flynn pleaded guilty in 2017 to lying about his contacts with Russia’s U.S. ambassador in the weeks before Donald Trump took office and Flynn became national security adviser. After agreeing to a plea deal with special counsel Robert Mueller, the former Defense Intelligence Agency chief cooperated with Mueller’s team for about a year and a half before switching to a more combative legal posture and eventually seeking to back out of his plea.

Following Flynn’s reversal, Attorney General William Barr moved to drop the case altogether, citing improprieties by the FBI decision-makers who ordered the interview of Flynn. But Barr’s move prompted outrage among Justice Department veterans and Democrats, who accused Barr of acting to protect a political ally of the president.

Though motions to dismiss brought by prosecutors are typically perfunctory, Sullivan pumped the brakes on the process, appointing an outside adviser to review whether he had discretion to deny the dismissal. Flynn’s team quickly accused him of bias, noting that Sullivan was sharply critical of Flynn during earlier rounds of proceedings and suggesting his drive to prolong the prosecution was inappropriate.

Flynn’s attorney, Sidney Powell, then filed a motion with the appeals court to force Sullivan to dismiss the case — a rare request for a so-called “writ of mandamus” that would override Sullivan’s plans.

But the appeals court said imposing a writ of mandamus is an extremely rare and delicate step reserved for cases in which a defendant has no other alternative. Here, Sullivan hadn’t even ruled on the motion to dismiss, the majority noted, before Powell sought to force his hand.

Powell decried the ruling Monday. “It is a disturbing blow to the #RuleOfLaw,” she wrote on Twitter.

The defense attorney, a prominent critic of the Mueller investigation, did not immediately indicate whether Flynn planned to ask the Supreme Court to step in or would now acquiesce in the hearing Sullivan had scheduled for last month but postponed.

The Justice Department backed Flynn’s effort to force Sullivan to dismiss the case, emphasizing that even if Sullivan conducts a hearing and seeks input on his options, he’ll have only one at the end of his search: dismissal. In the meantime, the Justice Department noted, Sullivan may ask a series of probing questions about internal processes that the court is not entitled to seek.

Rao said in her dissent that she was convinced Sullivan would do just that. And she described the case as a kind of zombie pursuit of Flynn that is senselessly grinding on with no one actually at the helm.

“In Flynn’s case, the prosecution no longer has a prosecutor,” she wrote. “Yet the case continues with district court proceedings aimed at uncovering the internal deliberations of the Department.”

However, the appeals court majority emphasized that Sullivan’s attorney, Beth Wilkinson, made clear that he had yet to decide what questions he might ask in a potential hearing. Rather, he might be satisfied by what the parties offer in written briefs. And she suggested that any assumptions about how Sullivan might rule are premature.

The only Republican-appointed judge to join the majority on Monday, George W. Bush appointee Thomas Griffith, is retiring on Tuesday. He lamented the fact that the ruling on Flynn’s case would be seen as driven not by “neutral principles” but by partisan affections.

“No doubt there will be some who will describe the court’s decision today in such terms, but they would be mistaken,” Griffith wrote in a concurring opinion. “Today we reach the unexceptional yet important conclusion that a court of appeals should stay its hand and allow the district court to finish its work rather than hear a challenge to a decision not yet made.”

Griffith went on to say that it would be “highly unusual” if Sullivan rejected the government’s motion, but that the judge should get to make his ruling and that there are “multiple avenues of relief” open to Flynn if he wishes to contest it.