During last week’s arguments on this issue, the Special Counsel took the position that there are a significant number of limitations on potential criminal prosecutions of ex-Presidents. These limitations would provide ex-Presidents with functional immunity, when they apply. They are not technically immunity from suit, which has its own distinct procedural aspects; when immunity applies in the technical sense, for example, the ex-President can raise this before trial and take an immediate (interlocutory) appeal if the argument is rejected. But these limitations would mean an ex-President could not be criminally convicted when they apply. Here’s a list of circumstances, according to the SC, in which an ex-President is functionally immune:

1. If the Attorney General gives the President advice that an action is legal, that provides an absolute defense. The SC, when asked this, responded very clearly that yes, it would be a “root violation of due process” to prosecute for that. At another point, the SC qualified this a bit by saying this would violate due process, “absent the kind of collusion or conspiracy that itself represented a criminal violation…”
2. The SC argued that Congress cannot criminalize presidential conduct in areas where doing so would interfere with an exclusive presidential power under Art. II or “that would prevent the president from accomplishing his constitutionally assigned functions.” As examples, he noted that pardon power; the veto power; the appointments power; the recognition of foreign governments; a small area of the commander-in-chief power, such as decisions about direction troops on the battlefield.
3. The SC’s position is that official acts can only be prosecuted when they are undertaken for “private gain.” This is a significant point which is easily missed. It means that disputes over whether a President acted lawfully in his official capacity cannot be turned into criminal prosecutions unless, in addition to the liability elements of the criminal statute, the President was also acting for private gain. To give a concrete example, US law makes it a domestic crime to commit certain “war crimes.” But even if some use of force by a President were to be considered a war crime by some international law standard, he could not be criminally prosecuted domestically, under the SC’s view, unless that use of force is found to have been undertaken for “private gain.” Abuse of office, under this view, means more than acting unlawfully; it means doing so for “private gain.” UPDATE: Marty Lederman has suggested to me that the SC meant to argue that private gain is sufficient to defeat immunity, but not necessary. I can see that possibility. But nothing in the SC’s argument made that point clearly. If he meant to say private gain was only one context in which immunity should not be recognized, that point did not come across.
4. The SC also argued all criminal law statutes should be understood to exclude from liability any action for which there is a “public authority” defense. This defense justifies conduct that is authorized by laws defining the duties or functions of (in this case) the President. The SC’s position, I think (less certain here) is that this defense does not turn on the subjective motives for which the P. acted. It turns on an objective characterization about the nature of his/her acts.
5. The SC acknowledged that criminal statutes applied to ex-Presidents must be construed so as to avoid serious constitutional questions about whether if applied they would interfere with a President’s ability to carry out his constitutionally assigned functions. This is different, and much narrower, than a principle that criminal statutes cannot be applied to an ex-President unless they specifically mention the presidency. But it is still a limitation on potential criminal liability for an ex-President; as Justice Sotomayor put it, this (and these other points) reflect “narrowing principles to the concept that the P. is subject to all criminal laws in all situations.”
6. The SC also stated that a politically driven prosecution of an ex-President would be selective prosecution and unconstitutional under Wayte v. United States.

Some of these limitations on presidential criminal accountability might overlap. And I’m not entirely clear about the SC’s position on some of them. At times, for example, he seems to suggest there can never be criminal liability for the P’s exercise of an Art. II power like the pardon power. At other times, he seems to suggest the P could be prosecuted for taking a bribe in return for granting a pardon (though the pardon would still stand).

There are two ways of looking at all these limitations. On the one hand, the SC is telling the Court there’s no need to adopt a sweeping immunity for all official acts, given that all these other limitations exist. From another perspective, these are acknowledgements that a President (or an ex-President) does have functional immunity from a significant range of potential criminal prosecutions.