Maybe you’re partially remembering the opinion (and it was just an opinion, not legal precedent) that a sitting President cannot be prosecuted for crimes? But the whole basis for that idea was that the proper remedy is impeachment to remove them from office, and upon being removed from office they can be tried for crimes by the DOJ (which they would no longer be the head of). This is what shielded Trump while he was in office, as his DOJ declined to prosecute him citing this logic as the reason.