Akhil Reed Amar posts an interesting article at Slate online magazine (link in title), "Mr. Jefferson, Meet Mr. Jefferson: What the framers would say about raids on congressional offices." He analyzes Thomas Jefferson's thoughts, both in the Constitution, Article I, Section 6 (dealing with privileges of the legislature) link and Fourth Amendment link dealing with search and seizure. He also looks at Thomas Jefferson's A MANUAL OF PARLIAMENTARY PRACTICE (1812) link. Prof. Amar concludes that the privilege of Article I, Section 6 against arrest of a sitting member of Congress does not insulate them against criminal investigation or arrest:
So, what did the Arrest Clause actually privilege? Basically, it insulated a sitting congressman from certain civil lawsuits brought by private plaintiffs seeking a court order that would physically "arrest" the defendant, with the effect (and perhaps purpose) of removing the congressman from the floor and thus disenfranchising his constituents. As Thomas Jefferson explained in his famed Manual of Parliamentary Practice: "When a representative is withdrawn from his seat by summons, the people, whom he represents, lose their voice in debate and vote." The theory was that one private litigant should not be allowed to undo the votes of thousands.
Prof. Amar analyzes more issues -- read the article. He, however, goes on to remind President Bush of a parallel historical incident in the past, where King Charles I pursued representatives of Parliament into their building, intending to arrest them. This rash act, says Amar, did much to set off the bloody English Civil War which cost King Charles his head.