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If one is not in office, he cannot be “Impeached.”
Impeachment is a term specific to making an accusation against an office holder.
Once out of office, they can be prosecuted in the normal fashion.
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You have to be right.
I remember Nixon was afraid of criminal charges more than removal and that made him decide to resign. He said it in a later interview (not David Frost).
Now all the pundits say you cannot put a sitting president in jail because he could not defend the country using the military in case of attack.
If true, Kim and Iran may be saving Trump’s bacon.