advocate; ideologist; ideologue
Refers to a person who actively supports or promotes a particular theory, principle, or ideology. Often used in formal or intellectual contexts.
He is known as an advocate of liberalism.
-ist (adherent)
Suffix attached to nouns denoting a belief system or principle to form a word for a follower or adherent. Very productive; e.g., 平和主義者 (pacifist), 菜食主義者 (vegetarian).
She is a vegetarian and doesn't eat any meat.
He calls himself a pacifist, but his actions are contradictory.
She is a devoted environmentalist.
He calls himself a democrat, but in reality he's authoritarian.
支持者 means a supporter who backs a person, policy, or cause, whereas 主義者 implies deeper ideological commitment to a system of belief.
Compound of 主義 (principle, doctrine) + 者 (person). The suffix usage is a direct calque from Western '-ist' formations.
totalitarian
communist