For those who don't know, the "speech and debate" clause in Article I, sec. 6 reads, in full, as follows "They [legislators] shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place."
As FairOpinion indicated, that provision says exactly what it sounds like (and case law e.g. United States v. Brewster consistent):
1) legislators, barring treason, felony, or breach of the peace, cannot be arrested while attending a legislative session
2) legislators may not be forced to account for what they said during a speech or debate, and neither their statements during the speech or debate nor the way they voted can be used against them.
Jefferson was not arrested while at a legislative session, nor was he asked to answer for anything he said during a legislative speech or debate. There are not even any allegations that he accepted any bribes on the floor during any speech or debate. The protections in the "Speech and Debate" clause therefore do not come within a country mile of protecting against the FBI's actions here.
I am particularly troubled by the inflammatory characterization of the FBI search as a "raid." The FBI had a warrant. When the executive branch gets a warrant, it necessarily seeks a neutral and detached assessment by the judicial branch that the proposed search is proper and consistent with the law. Of course, Congress can't make the executive branch (and implicitly President Bush) look like a dangerous, imperialistic power unless they ignore details like that. And as long as Congress is in a self-aggrandizing mood, we might as well expect next week's drama to be a legislative enactment overturning Marbury v. Madison and the concept of judicial review.
As for Congress' gripe that this search was "unprecedented," that is the among the most woefully inadequate attempts at a legal rationale I have ever heard. Even if other presidents had an unwritten "gentleman's agreement" to wink and nod at criminal behavior, so long as the evidence remained buried on Capitol Hill, this president has no obligation to follow suit. Nothing in the Constitution indicates that governmental powers are like muscles and "atrophy" from nonuse.
Despite the fact that the legal arguments that the executive branch did anything wrong are on weak legal footing, this is a fact scenario that has not arisen before. We can reasonably expect Jefferson to file a motion to suppress evidence in the event he is charged (I'm not sure if that has happened yet). It also would not be suprising if he started a lawsuit over this search. The forty-five day waiting period will allow the executive branch a reasonable amount of time to see what sort of legal challenges materialize. If the court does find that some of the documents were improperly taken (e.g. if there are copies of floor speeches) the executive branch will be able to tailor any subsequent investigation or prosecution to avoid use of those documents.
If this "wait and see" approach is what President Bush has in mind, then that is a reasonable strategy. By minimizing the risk that any protected documents will be used against Jefferson, the chance of tainted evidence poisoning the entire proceedings diminishes. Obviously, if Jefferson is convicted of wrongdoing, we want that conviction to stand, not to be overturned on a technicality.
In some cases, "due process" or "equal protection" issues may come into play. Many places have laws which haven't been enforced for many decades because technological or sociological changes have rendered them obsolete. Despite (or perhaps because of) the lack of enforcement, however, nobody has bothered to remove such laws from the books.
If someone were charged with violating such a law, the fact that the law had not been enforced for ages, and that nobody made any particular effort to comply with it, could be a basis for acquittal, especially if there was any reasonable possiblity that the prosecution of a particular case was motivated by any illegitimate factors.
The fact that a law hasn't been enforced is no bar to prosecution if the reason for the lack of enforcement is that the law has been well-enough respected that enforcement has not been necessary. In such a case, the lack of enforcement would not favor the defense but might, if anything, favor the prosecution.
More generally, a law that society perceives as not existing may be rendered unenforceable by such perception. A law that is well-ingrained into society may not.