noun
supporter; sponsor; patron; backer
A person or organization that provides support, often financial or influential, to a person, group, or cause.
彼は地元の政治家の後援者だ。
He is a supporter of the local politician.
このイベントは多くの後援者のおかげ
This event was made possible thanks to many sponsors.
スポンサー is a direct loanword for 'sponsor', commonly used in commercial or media contexts, whereas 後援者 can carry a broader sense of patronage.
Compound of 後援 (こうえん, 'support, backing') and 者 (しゃ, 'person'). The term is a straightforward Sino-Japanese formation.