They were not searching for documents so much as they were money. Do you think privileged docs were hidden in the freezer with 90 grand of marked money that was also caught on tape being excepted as a bribe? What, we should just let that ride and say, oh, hell we can't search a congressmans office, he's above the law? BS. The constitution does not exempt senators and representatives from search when it pertains to a felony.
Well, they weren't searching for privileged documents. They were searching for evidence of a crime- and had good cause.
However they did seize and examine clearly privileged documents because the evidence was hidden among them.
The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States. They 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. Article 1 Section 6, Constitution for the United States.