Contempt of Congress is the act of obstructing the work of the United States Congress or one of its committees. While historically the bribery of a Senator or Representative was considered "contempt of Congress," in modern times a person must refuse to comply with a subpoena issued by a Congressional committee or subcommittee - usually seeking to compel either testimony or documents - in order to be considered in "contempt of Congress."
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Statutory proceedingsFollowing a contempt citation, the presiding officer of the chamber is instructed to refer the matter to the U.S. Attorney for the District of Columbia<2>; according to the law it is the "duty" of the U.S. Attorney to refer the matter to a grand jury for action.
The criminal offense of "contempt of Congress" sets the penalty at not less than one month nor more than twelve months in jail and a fine of not less than $100 nor more than $1,000. Those penalties are enforced upon conviction, even if the Congress which initiated the contempt citation has expired.
The statutory procedure has generally been used by Congress since 1935. While the law pronounces the "duty" of the U.S. Attorney is to impanel a grand jury for its action on the matter, dispute exists over whether the Congress can properly compel the U.S. Attorney to take this action, as the U.S. Attorney is a member of the Executive Branch who ultimately reports to the President. (The Courts have been reluctant to decide this question, claiming it is a "political question" for resolution by the elected branches of government.)
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http://en.wikipedia.org/wiki/Contempt_of_Congress