It is fairly complicated. Should Obama be imprisoned for murder of US citizens in drone strikes? Should Bush be held liable for the incompetence in allowing 9/11? Should Clinton be held responsible for deaths in Kosovo for violation of the war powers act?
Presidents do seem to have immunity with regard to actions as a President. Creating a ruling that protects presidential action and allows criminal prosecution for other actions is going to be difficult. There’s also the matter of if actions done with immunity can be used as evidence against a president.
There is some existing precedent for distinguishing the President from the Candidate during elections. This could be used as a basis for a ruling Trump isn’t immune, but it’s hardly definitive.
It’s going to take more than an “I know it when I see it” ruling.