He should use the US military if necessary. Arrest a few judges as well, to send a message to the rest of them. This is Attempted Coup chap. 2. Chapter one, The Mueller Witchhunt, left many disappointed. So we get congressmen engaging in a clearcut violation of separation of powers. They do not run the IRS, any more than they can direct policy at any other agency. The president does and is duty-bound to resist attempts by other Branches to encroach on his authority. The Constitution gives him the last word in these situations, by virtue of his being Commander in Chief. In the end, power rests those with able to grasp it. It's a lesson the left knows by heart but republicans seem unable to grasp it.