Refers to a person who asserts or advocates a particular opinion, right, or claim. Often used in formal or argumentative contexts.
He is known as an advocate for environmental protection.
Proponents of that policy emphasized economic growth.
支持者 is a supporter or backer, often less active in argumentation than a 主張者.
Compound of 主張 (assertion, claim) + 者 (person). The reading follows standard on'yomi.