Conservators are those that work towards the conservation of things.
Edit: conservation and conservatism are 2 very different things. To work towards conservation of something is not going to be directly tied to political backgrounds: It is not because someone is a socialist or liberal, that they will not want to conserve some things. Everyone will probably have some things (nature, monuments, traditions, …) that they want to conserve for future generations.
Theodore Roosevelt was considered domestically progressive, internationally conservative. The encyclopedia Britannica has this to say on Teddy: “By 1906 he was the undisputed spokesman of national progressivism and by far its best publicity agent.”. https://www.britannica.com/place/United-States/Theodore-Roosevelt-and-the-Progressive-movement
What do you call a progressive who wants to conserve a piece of nature? Still a progressive, because progressive/conservative terminology is about political philosophy/ideology, not about conservation. Teddy Roosevelt was a progressive politician who also was a conservator of nature.