Washington (CNN) The husband of White House counselor Kellyanne Conway is arguing that the President's decision to instate Matthew Whitaker as acting attorney general is "unconstitutional" and "illegal."

On Wednesday, President Donald Trump requested Attorney General Jeff Sessions' resignation and announced Whitaker, who was Sessions' chief of staff, would take charge of the Department of Justice until a permanent replacement is nominated at a later date.

George Conway, Kellyanne Conway's husband, slammed the move in an op-ed published Thursday in the New York Times

"A principal officer must be confirmed by the Senate. And that has a very significant consequence today. It means that Mr. 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," Conway argued in the op-ed, co-written with Neal Katyal, former acting solicitor general under former President Barack Obama.

They added, "And it means that anything Mr. Whitaker does, or tries to do, in that position is invalid."

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