A campaigner is someone who works, often for a long time, to bring about a social or political change. They might collect signatures, knock on doors, speak at meetings, organise a march, or keep writing to people in power. You can be a campaigner for cleaner air, for animal welfare, against a new road, and so on. It is not only for famous leaders: a quiet person who shows up every weekend is a campaigner too. The word feels public and a little serious; you would not use it for a family argument at dinner. Though actually, in older British English an “old campaigner” could also mean someone who has been through many hard jobs or wars and knows how to cope — that sense comes from military campaigns, and you still hear it now and then.