Impeach Obama
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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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The idea of impeachment goes back to common law and includes undefined abuse of power or corruption as a cause. Such an abuser could escape with perks, pension, and eligibility still intact by merely resigning to avoid impeachment and removal and the handicaps that come with it.
Somewhat similar to the Execurtive’s broad power of pardon, conditions and limits on impeachment (and guilt) are self-determined by the Legislative branch.