FT Professional Glossary
Stalwart
A loyal supporter who does a lot of work for an organisation, especially a political party
Example usage from an FT article:
She has been a stalwart of Republican politics for decades, reportedly first working for House member Jack Kemp, an ex-teammate of Summerall’s, before working on Ronald Reagan’s 1980 presidential campaign.
Excerpt from "Donald Trump chooses Susie Wiles as White House chief of staff"