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1.Was the F.B.I. raid on Rep. Jefferson’s office legal or illegal?
From the looks of it appears that it is legal. From the Cato Institute:
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Jefferson is under investigation for bribery. Congress itself has expressly made it a crime for members of Congress to accept bribes (18 U.S.C. s. 201). The Supreme Court has held (in United States v. Johnston, 383 U.S. 169 (1966)) that the Speech and Debate Clause does not absolutely immunize members of Congress from prosecution under a valid bribery statute (enacted, remember, by Congress).
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2.Does this raid set a good/bad precedent?
It sets a valid precedent that as a member of congress, you will darn well be held accountable for what you do.
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3.Was the raid yet another attempt by the Bush Administration to usurp power, thereby shifting the balance of power in favor of the executive?
This guy was filmed taking money and reportedly had $90,000 stuffed in a freezer. If the president is hunting down corruption, then I support him.
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4. Given that other Congressmen have faced allegations, what "at random"(?) factor made Jefferson the first member of Congress to have his office searched?
Once again, this deals with bribery. The only reason why other elected members haven't had their offices searched is that they probably hid the goods elsewhere.