opposer
Definitions
A person who actively resists or disagrees with a person, group, or idea.
One who acts in opposition to another in a contest, debate, or conflict.
Examples
The senator was a vocal opposer of the new tax legislation throughout the entire debate.
As a long-time opposer of the development project, she attended every town hall meeting to voice her concerns.
Synonyms
adversary antagonist opponent challenger competitor enemy detractor