Turncoat

A turncoat is a person who shifts allegiance from one loyalty or ideal to another, betraying or deserting an original cause by switching to the opposing side or party. In political and social history, this is distinct from being a traitor, as the switch mostly takes place under the following circumstances:

• In groups, often driven by one or more leaders.
• When the goal that formerly motivated and benefited the person becomes (or is perceived as having become) either no longer feasible or too costly even if success is achieved.