To a limited extent, they are.
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.
Well .. one of the categories is "felony" which I believe would apply to the congress person who takes thousands of dollars in BRIBE MONEY.
So .. the "raid" was not illegal at all!!!!!