turncoat

n. traitor, person who switches sides (to an opposing political party, ideal, etc.)

For other uses, see Treason (disambiguation) or Traitor (disambiguation). In law, treason is the crime that covers some of the more serious acts of disloyalty to one's sovereign or nation. A person who commits treason is known as a traitor. Oran's Dictionary of the Law (1983) defines treason as: "...[a]...citizen's actions to help a foreign government overthrow, make war against, or seriously injure the [parent nation]." In many nations, it is also often considered treason to attempt or conspire to overthrow the government, even if no foreign country is aided or involved by such an endeavour.
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Noun
1. a disloyal person who betrays or deserts his cause or religion or political party or friend etc.
(synonym) deserter, apostate, renegade, recreant, ratter
(hypernym) quitter

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

Synonyms and related words:
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