He is a supporter of the local politician.
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後援者 emphasizes ongoing financial or organizational backing, whereas 恩人 focuses on the personal gratitude one feels toward the helper.
後援者 is a native Japanese term emphasizing moral or organizational backing, whereas スポンサー stresses financial support in exchange for advertising.
後援者 is a neutral, formal word for a supporter or backer, while 金蔵 has a more colloquial, sometimes slightly cynical tone of being used as a money source.
Compound of 後援 (こうえん, 'support, backing') and 者 (しゃ, 'person'). The term is a straightforward Sino-Japanese formation.