Republicans will bleat the installment of Whitaker is fine. They are wrong. Trump's appointment of Matthew Whitaker as Acting Attorney General is unconstitutional. More simply, it is illegal.
We all better be versed in the law, and how to explain it clearly.
Here it is:
Art. II, Sec. 2, Clause 2 of the Constitution clearly states that principal officers of the United States must be confirmed by the Senate.
“Principle offers” are those that report directly to the President. The Attorney General reports directly to the President. Therefore, the Attorney General is a “Principle Officer” and must be confirmed by the Senate.
Or even more simply:
Whitaker reports only to the president and to no one else, so he is a principal officer and as such needs Senate confirmation.
Whitaker has not been confirmed by the Senate.
Whitaker’s appointment is unconstitutional. It is illegal.
In summary:
The Constitution clearly states that principal officers of the United States must be confirmed by the Senate. Whitaker reports only to the president and to no one else, so he is a principal officer and as such needs Senate confirmation. He does not have Senate confirmation. His appointment is illegal.
An important corollary:
Because Whitaker’s is an invalid AG, anything Whitaker does, or tries to do, in that position is invalid.
Finally, how to proceed:
Whitaker will do something that someone with standing can challenge. Standing means you’ve suffered an actual or immanent harm. That will happen very soon. Such a person will sue Whitaker for not having the authority of the Attorney General because the Senate didn't confirm him.
Given Whitaker’s thin credentials, his vocal criticism of Mueller, even some Republicans are unhappy about the move. So, stay tuned.
[Regarding Session’s forced resignation: As far as narrow argument presented here, Session could have been actually resigned, or been fired, and the illegality of Whitaker’s installment remains the same.]