campaigner

From campaign (verb) +‎ -er (agent noun suffix), or campaign (noun) +‎ -er (occupational suffix).[1]

campaigner (plural campaigners)

1. A person who has served in a military campaign.
2. (by extension) 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.
   • 2013 June 21, Karen McVeigh, “US rules human genes can't be patented”, in The Guardian Weekly, volume 189, number 2, page 10:

     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.

4. Someone with experience in a certain field.

• recampaign