A supporter or adherent — someone who endorses or advocates for an idea, movement, or person (often used in political or ideological contexts).
新党の支持者が街頭で演説を聞いている。
Supporters of the new party are listening to the speech on the street.
彼女はこの理論の熱心な支持者だ。
She is an ardent supporter of this theory.
支持者を増やすためにキャンペーンを行う。
They are running a campaign to increase supporters.