Oath Required By The Constitution And By Law To Be Taken By Senators:
"I, A__ B__, do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter: So help me God." (5 U.S.C. 3331.)
United States Senate Oath of Office
It doesn't look that complicated to me. It seems to me that breach of this oath constitutes unfitness for the job, even in the absense of proof of any wilful treason or treachery or perjury. People don't have to get fired, they terminate themselves by failure to perform required job functions. Incompetence or insubordination is cause for dismissal!
Ride McCain and his ilk out of town on a rail and for severance pay, give a month's worth of peanut brittle.