The Constitutional Convention of the late 1700s placed the Speech or Debate Clause into Article I, Section 6, Clause 1, of the U.S. Constitution. The clause protects members of Congress from prosecution for their activities related to the legislative process. Legislative activities within the Senate or House that may not be part of a prosecution against a member include speeches and debates, preparing committee reports, voting, conducting committee hearings, and any other task required by the nature and execution of the office.
Legislators are even protected from prosecution when accused of violating another individual's constitutional rights. The Supreme Court has held, "Legislators are immune from deterrents to the uninhibited discharge of the legislative duty . . . for the public good." The protection, although not absolute, extends to congressional aides if their duties involve legislative activities a member of Congress would perform.
Since protection is limited only to conduct which is part of the legislative process, legislators' remarks published in newsletters, made in press releases, or on television are not immune from prosecution. Taking a bribe to influence legislation also falls outside the immunity. In striking a balance, the clause can not be interpreted so broadly as to allow erosion of the legislature's integrity.
First of all, if you have enough votes in the House and Senate you can “expel” members ~ whereupon whatever traitorous statements they make become quite punishable.
If their financial records were gone over with a microscope, looking for evidence of bribery, kickbacks, inurement, or other prosecutable offenses, how many would come out as being utterly clean?