Section I, Article VIIII of the U.S. Constitution - "No Title of Nobility shall be granted by the United States: And no Person holding any Office of Profit or Trust under them, shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince or foreign State."
It is a conflict of interest for an elected leader in the United States to hold any another office. We elect our leaders to work for and answer to the good people of the United States of America. When an elected official is subject or obligated to the desires of a foreign body or amassed foreign bodies, he/she runs the risk of acting in a manner or making decisions not commensurate with the wishes of the people.