It ought to be that Presidents, and Past Presidents, are immune from criminal charges for things they did or said in office as part of their “official duties”, unless first impeached and convicted by the Senate for the same charges. Or at least something close to that (there are a few limits to the Congressional immunity provided by the debate clause, mostly things limiting it to immunity for actions, speech that are part of their official duties. If trying to charge the President for something under the loophole that it wasn’t part of his official duties that should require a high level of proof that it wasn’t an official action.
bttt