partisan
n. countablen. a person who strongly supports one specific group, party, or idea. You use this when someone is so loyal to their side that they might not be fair to others.
n. a committed member or strong supporter of a political party, faction, or cause. Often carries a pejorative nuance suggesting biased or uncritical loyalty.
He is a lifelong partisan who always votes for his party.
The news channel was criticized for hiring a known partisan to lead the debate on economic policy.
In an era of deep political division, even neutral administrative decisions are often viewed through a partisan lens by a skeptical public.
From French partisan, from Italian partigiano (“defender of a party”), from parte (“part”). Doublet of partigiano. Attested in English from the late 15th century in the noun sense of "party adherent", and in related adjective senses from the 16th century. The "guerrilla fighter" sense influenced by Serbo-Croatian partizan, Russian партиза́н (partizán), from the same source. The sense of "guerrilla fighter" is from c. 1690. The adjective in the military sense dates from the early 18th century.
From French partizaine, from Middle French partizaine, partisanne etc., from Italian partigiana, related to Etymology 1 above (apparently because it was seen as a typical weapon of such forces).
Often used as an attributive noun to modify other nouns, such as in 'partisan politics' or 'partisan divide'.