There were many opponents to the plan.
彼は議論の中で反対者の意見をよく聞いた。
He listened carefully to the dissenter's opinion during the debate.
相手 is a neutral term for 'partner' or 'opponent' in a game, match, or interaction, without the connotation of dissent that 反対者 carries.
主義者 is someone who advocates for a doctrine, while 反対者 is someone who opposes it.
Compound of 反対 (opposition) + 者 (person). The reading is standard on'yomi.