Special Counsel Robert Mueller could use a legal concept known as "constructive discharge" to challenge the appointment of Matt Whitaker, the acting Attorney General, by arguing that Attorney General Jeff Sessions was forced out as opposed to voluntarily leaving, reports Bloomberg, citing a former federal prosecutor.

Mueller could argue in court that Trump effectively fired Sessions after months of verbal abuse, a legal concept known as a constructive discharge, said Renato Mariotti, a former federal prosecutor. Under the Federal Vacancies Reform Act, Trump can appoint an acting official without Senate confirmation if he replaces someone who has been incapacitated or resigned. It doesn’t apply if the previous official was fired. -Bloomberg

Whitaker was appointed to run the DOJ after Sessions submitted his resignation Wednesday at Trump's request. While Sessions had recused himself from the Trump-Russia probe, Whitaker will now control oversight of the investigation - a duty which has fallen on the shoulders of Deputy Attorney General Rod Rosenstein - despite the fact that he himself was involved in the FISA warrant process to spy on the Trump campaign.

Sessions' resignation letter begins with "At your request," making it unambiguous that Trump fired him.

"The question is whether he was constructively fired, which means he didn’t resign from his post," Mariotti said. "I don’t know the answer as to how the courts would view that."

Challenging Whitaker's appointment "could be Mueller himself," said Mariotti, adding "That would be one obvious person."

Legal experts agree it would be difficult to remove Whitaker from a post he can hold for seven months under the law. He can’t be appointed permanently, and Trump said he would appoint someone at a later date. -Bloomberg

"It’s not clear whether a firing would allow Trump to appoint him as an interim," said former federal prosecutor Barbara McQuade, who teaches law at the University of Michigan. If Sessions voluntarily resigned, "it's permissible for Trump to make this interim appointment."

"I don’t see any reason why Whitaker would not be the one to supervise the Mueller investigation and take it out of the hands of Rod Rosenstein," she added.

Rosenstein appeared at the White House on Wednesday for a previously unscheduled appointment.

Meanwhile, Bloomberg notes that special counsels can be removed under the law for "misconduct, dereliction of duty, incapacity, conflict of interest, or for other good cause." Whitaker is on record saying that if Mueller investigates the Trump family finances beyond anything to do with Russia, "that goes beyond the scope of the special counsel."