(n.) One who forsakes his party or his principles; a renegade; an apostate.

Benedict Arnold Brutus Judas Judas Iscariot Judas-like Quisling Trojan-horse Vicar of Bray apostate archtraitor backslider betrayer betraying bolter cockatrice collaborationist collaborator conniver conspirator conspirer convert defector deserter double agent double-crosser double-crossing double-dealer fifth columnist fifth-column informer intrigant intriguer machinator mugwump plotter proselyte quisling quislingistic rat recidivist recreant renegade renegado renegate reversionist runagate schemer schismatic seceder secessionist separatist serpent snake spy straggler strikebreaker tergiversant tergiversator timeserver traitor traitorous treasonable treasonist treasonous trimmer turnabout turntail two-timing