campaigner

noun

1. A person who has served in a military campaign.
2. (broadly)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.”

   “The financial campaigner James Daley has launched a £1.5bn class action lawsuit against Apple over its mobile phone wallet, claiming the US tech company blocked competition and charged hidden fees that ultimately harmed 50 million UK consumers.”

4. Someone with experience in a certain field.