George Conway, White House counselor Kellyanne Conway’s husband, argued Thursday that President Trump’s appointment of Matthew Whitaker as acting attorney general is unconstitutional and any action he takes while leading the Justice Department will be “invalid” because he lacks Senate confirmation.

Conway, who co-authored an op-ed with Neal Katyal in the New York Times, said the Appointments Clause of the Constitution states that principal officers, who are defined as someone who reports only to the president, must be confirmed by the Senate.

“It means that President Trump’s installation of Matthew Whitaker as acting attorney general of the United States after forcing the resignation of Jeff Sessions is unconstitutional. It’s illegal. And it means that anything Mr. Whitaker does, or tries to do, in that position is invalid,” Conway and Katyal write.

“Mr. Whitaker’s installation makes a mockery of our Constitution and our founders’ ideals,” they added.

Trump appointed Whitaker on Wednesday after firing Jeff Sessions. Whitaker, who served as Sessions’ chief of staff, will oversee the Justice Department until a replacement is nominated by Trump and confirmed by the Senate.

Experts debate whether Whitaker can be acting attorney general without Senate confirmation. One argument in support of the appointment relies on the Federal Vacancies Reform Act of 1998, which says a person may serve “no longer than 210 days beginning on the date the vacancy occurs,” or “once a first or second nomination for the office is submitted to the Senate, from the date of such nomination for the period that the nomination is pending in the Senate.” If the Senate rejects a president’s nominee, then the temporary replacement can serve for another 210-day period.

Conway and Katyal said that argument is flawed, citing the Supreme Court ruling last year that Lafe Solomon improperly served as the general counsel of the National Labor Relations Board because he was not confirmed by the Senate. Former President Barack Obama appointed Solomon to the position in 2010.

“We cannot tolerate such an evasion of the Constitution’s very explicit, textually precise design. Senate confirmation exists for a simple, and good, reason. Constitutionally, Matthew Whitaker is a nobody. His job as Mr. Sessions’s chief of staff did not require Senate confirmation,” they write. “For the president to install Mr. Whitaker as our chief law enforcement officer is to betray the entire structure of our charter document.”

Because Whitaker lacks Senate confirmation, they added, his character and ability to enforce the law impartially has not been scrutinized.

“The public is entitled to that assurance, especially since Mr. Whitaker’s only supervisor is President Trump himself, and the president is hopelessly compromised by the Mueller investigation,” the duo write.

With Sessions’ departure, Whitaker is likely to oversee special counsel Robert Mueller’s investigation into Russian interference in the 2016 election, taking over for Deputy Attorney General Rod Rosenstein, who had been in charge of the investigation’s oversight since Sessions recused himself last year.

Whitaker has been critical of the Mueller investigation, writing an op-ed for CNN in 2017 headlined, “Mueller’s investigation of Trump is going too far.”