Not impeachment...
House Practice: A Guide to the Rules, Precedents and Procedures of the House
A Member of Congress is not a ``civil Officer'' within the meaning of the impeachment provisions of the Constitution. 3 Hinds Sec. Sec. 2310, 2316.
Expulsion...
Article I, Section 5, Clause 2:
Each House may determine the Rules of its Proceedings, punish its Members for disorderly Behaviour, and, with the Concurrence of two thirds, expel a Member.