A person sharing responsibility for a political party's discipline and tactics

One word substitute is Whip.

Statesman: a skilled, experienced, and respected political leader or figure.
Diplomat: an official representing a country abroad.
Whip: an official of a political party appointed to maintain parliamentary discipline among its members, especially so as to ensure attendance and voting in debates.
Defector: a person who has abandoned their country or cause in favour of an opposing one.