He said there are only three ways someone can become acting attorney general: if they are the deputy attorney general and the president signs an executive order to make them acting attorney general, if they are already at the Justice Department in a job that both required and received Senate confirmation or if the Senate in in recess, they can be a recess appointment.

He said none of those apply in this case, as Whitaker is not deputy attorney general, he did not receive Senate confirmation for his current role at the DOJ and the Senate is not in recess.

"So with deference and respect to what the president's trying to do -- he has every right to want whoever he wants to run the Justice Department -- he has chosen someone who does not qualify under the law to be the acting attorney general of the United States," Napolitano said.