A supporter or proponent — someone who agrees with and backs an opinion, proposal, or cause.
その政策には多くの賛同者が集まった。
Many supporters gathered for that policy.
賛同者を増やすために、丁寧に説明を続けた。
We continued to explain carefully to increase the number of supporters.
会議には賛同者と反対者の両方が出席した。
Both supporters and opponents attended the meeting.