campaigner

noun

Etymology: From campaign (verb) + -er (agent noun suffix) or from campaign (noun) + -er (occupational suffix).

1. A person who has served in a military campaign.
2. A military veteran.
3. A person who campaigns for a person running for political office or works, or supports, in an organised and active way towards a goal; an activist.

   “The US supreme court has ruled unanimously that natural human genes cannot be patented, a decision that scientists and civil rights campaigners said removed a major barrier to patient care and medical innovation.”

   “It is remembered unfondly by politicians, communities and campaigners as the document that heralded widespread reductions in the size of the railway.”

4. Someone with experience in a certain field.