PARTISAN is validvalid

a firm supporter of a person, party, or cause [n PARTISANS] : PARTISANSHIP [n]

76,577 of 196,601 in NWL2023·also valid in CSW24

biased [adj PARTISANLY] / an adherent, esp. a blind or unreasoning adherent, of a party or a faction, also PARTIZAN [n -S]