Lavin says that any past president may be charged , so President trump can go back on Obama, Clinton and Bush?
500?
impeachment results in losing office and, optionally, not being able to again hold office - retroactive impeachments are to impose the latter. WWG1WGA :)
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In theory, yes.
But it is never going to happen unless they find a direct, smoking-gun paper trail of bribes; for example a personal check deposited in Obama's bank account, signed by the head if Hezbollah, and with the reference note added: "For services rendered in quashing DEA investigation."
Otherwise no sane President would ever want to set the precedent of prosecuting their predecessor(s) for alleged crimes committed in office.