noun
Refers to a person who asserts or advocates a particular opinion, right, or claim. Often used in formal or argumentative contexts.
彼は環境保護の主張者として知られている。
He is known as an advocate for environmental protection.
その政策の主張者は、経済成長を強調した。
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.