The basic idea is that you can prosecute bribery simply by investigating whether the member of Congress illegally took money in exchange for a promise to do some official act. You don't then need to investigate what the member of Congress actually did as a legislator, and that is the part of the investigation that would give rise to a Speech and Debate clause problem.
No branch or two branches of government working in concert, may intrude upon the fundamental rights of the other.
While the Speech and Debate Clause has been expressly held not to shield Senators or Representatives against bribery charges, Johnson v. United States, 383 U.S. 169 (1964), it does impose significant limits on the type of evidence that can be used to prove such an offense. The Clause broadly protects members of Congress "against inquiry into acts that occur in the regular course of the legislative process and into the motivation for those acts," United States v. Brewster, 408 U.S. 501, 525 (1972), and "precludes any showing of how [a member of Congress], acted, voted, or decided." Id. at 527."
To exercise exclusive legislation in all cases whatsoever, over such District (not exceeding ten miles square) as may, by cession of particular states, and the acceptance of Congress, become the seat of the government of the United States, and to exercise like authority over all places purchased by the consent of the legislature of the state in which the same shall be, for the erection of forts, magazines, arsenals, dockyards, and other needful buildings
Gonzales said at a Justice Department news conference. "I ... and the department have a great deal of respect for the Congress as a coequal branch of government ..."
At some point, this Administration seems to have forgotten that the Constitution is the supreme law of the land, and not just one of many factors that should be balanced against other factors.