Well the Constitution says:
Section. 6. 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.
It's generally been interpreted to mean they can say anything about anybody and not be sued for it. However most cases have involved other political or public figures. Then again his words have give aide and comfort to the enemies of the United States. That's treason, for which even CongressCritters of the liberal persuasion can be charge, tried and convicted.