Not just a current president charging a former one.....any DA. We have about 29,000 district attorneys in this country. Some in very blue districts, but some in very red districts too. If the president is subject to criminal charges for his acts as president....even after leaving office, how much does anybody want to bet partisan DA’s in highly partisan districts will start coming up with wild legal theories to charge their political opponents in districts where those opponents have almost no hope of obtaining a fair trial?
The constitution is quite clear about the president. He can be impeached only.....ie he can be charged by the House and tried in the Senate. There is no role for the judiciary.
They have multiple actions they can be charged for.
Clinton violated the Posse Comitatus Act at Waco. That is not a convoluted sorta, kinda, maybe thing. Federal military personnel were being used to enforce domestic policies. And a bunch of people, including civilians died.