noun
party defector; party secessionist
Refers to a person who leaves or defects from a political party. Often used in news reports.
その離党者は新党を結成した。
The party defector formed a new party.
脱党者 is a near-synonym, but 離党者 may sound slightly more formal or neutral, while 脱党者 can imply a more dramatic break.
Compound of 離党 (leaving a party) + 者 (person).