I say impeach Harkin for violating his oath of office. This is what he swore:
I 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.I want you to think about that underlined part: what Harkin proposes in this story is changing the definition of domestic enemies.
Saying it might be right to do it is probably not impeachable -- actually doing it would be impeachable. Unless he wants to change the oath of office, too -- which would be part for the course, I guess.