Are you not understanding that there is NO impeachment of a U. S. Senator authorized? That is the issue. Nothing else.
Are you sure? Are you really sure?
See U.S. Constitution Article I, Section 5, and specific for the Senate, the Select Committee on Ethics. See the Committee on Standards of Official Conduct, initiating an investigatory committee. Voting to expel requires the concurrence of two-thirds of the members present and voting.
The U.S. Senate can expel, and has a prior history of expelling sitting members.
http://www.senate.gov/reference/index_sub_items/Expulsion_vrd.htm