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Persons Subject to Impeachment
During the debate in the First Congress on the ''removal'' controversy, it was contended by some members that impeachment was the exclusive way to remove any officer of the Government from his post, but Madison and others contended that this position was destructive of sound governmental practice, and the view did not prevail. Impeachment, said Madison, was to be used to reach a bad officer sheltered by the President and to remove him ''even against the will of the President; so that the declaration in the Constitution was intended as a supplementary security for the good behavior of the public officers.'' The language of Sec. 4 does not leave any doubt that any officer in the executive branch is subject to the power; it does not appear that military officers are subject to it nor that members of Congress can be impeached.
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Impeachable Offenses
The Convention came to its choice of words describing the grounds for impeachment after much deliberation, but the phrasing derived directly from the English practice. The framers early adopted, on June 2, a provision that the Executive should be removable by impeachment and conviction ''of mal-practice or neglect of duty.'' The Committee of Detail reported as grounds ''Treason (or) Bribery or Corruption.'' And the Committee of Eleven reduced the phrase to ''Treason, or bribery.'' On September 8, Mason objected to this limitation, observing that the term did not encompass all the conduct which should be grounds for removal; he therefore proposed to add ''or maladministration'' following ''bribery.'' Upon Madison's objection that ''so vague a term will be equivalent to a tenure during pleasure of the Senate,'' Mason suggested ''other high crimes and misdemeanors,'' which was adopted without further recorded debate. The phrase in the context of impeachments has an ancient English history, first turning up in the impeachment of the Earl of Suffolk in 1388.
Treason is defined in the Constitution; bribery is not, but it had a clear common-law meaning and is now well covered by statute. High crimes and misdemeanors, however, is an undefined and indefinite phrase, which, in England, had comprehended conduct not constituting indictable offenses. In an unrelated action, the Convention had seemed to understand the term ''high misdemeanor'' to be quite limited in meaning, but debate prior to adoption of the phrase and comments thereafter in the ratifying conventions were to the effect that the President at least, and all the debate was in terms of the President, should be removable by impeachment for commissions or omissions in office which were not criminally cognizable. And in the First Congress' ''removal'' debate, Madison maintained that the wanton removal from office of meritorious officers would be an act of maladministration which would render the President subject to impeachment. Other comments, especially in the ratifying conventions, tend toward a limitation of the term to criminal, perhaps gross criminal, behavior. While conclusions may be drawn from the conflicting statement, it must always be recognized that a respectable case may be made for either view.
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http://supreme.lp.findlaw.com/constitution/article02/18.html