turncoatturn‧coat /ˈtɜːnkəʊt $ ˈtɜːrnkoʊt/ noun [countable] 
someone who stops supporting a political party or group and joins the opposing side: 
Casson was publicly criticized as a turncoat and a traitor.
someone who stops supporting a political party or group and joins the opposing side: Casson was publicly criticized as a turncoat and a traitor.