[Gustoo]

"The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any Subject relating to the Duties of their respective Offices, and he shall have Power to grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment."

They may wish to officially impeach him so that his pardons can be negated. Maybe all of them depending on how the court decides it, since I am guessing there is no precedent here. This would make egregious pardoning less of a problem since such pardons are nullified by successful impeachment. Also these pardons are for offenses against the United States (like going to war) so there seems to be lots of other avenues to prosecure people he has pardoned.