Had the U.S. Supreme Court not ruled that the United States was bound by the Fourth Geneva Conventions in the case, Hamdan v. Rumsfeld (2006), there would be no reason to go after the Bush Administration. I hope I have it correct, but it is this Geneva Convention, Protocol II (that dealt with combatants in non-international armed conflicts) that needed ratification by Congress to be law. Elements of such treatment were covered by the Uniform Code of Military Justice. Since there was no ratification, the court as usual, bypassed the legislative process and the will of the people.
I think the approach the lefties will take is right up the middle, defining waterboarding as torture, which is illegal under U.S. law. I know it sounds nuts: how could you prosecute a lawyer for writing an opinion that the technique as approved for the CIA is not torture and is legal for conspiracy to engage in torture??? Well, think about an out of control political prosecutor (a la Fitz), a D.C. jury and an Obama appointee Judge. Talk about a kangaroo court.