secessionist

adj. pertaining to a withdrawal from an alliance or organization; pertaining to one who supports withdrawal from an alliance or organization

n. one who supports withdrawal from an alliance or organization

Secession (derived from the Latin term secessio) is the act of withdrawing from an organization, union, or political entity. It is not to be confused with succession, the act of following in order or sequence. Typically there is a strong issue difference that drives the withdrawal. Though the term is often associated with full civil war, there are different degrees of secession, some as minor as a particular neighborhood seeking to become a separate municipality from a larger city, while still maintaining greater local or national ties.
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Noun
1. an advocate of secessionism
(hypernym) advocate, advocator, proponent, exponent

(n.)
One who upholds secession.
  

(n.)
One who holds to the belief that a State has the right to separate from the Union at its will.
  

Synonyms and related words:
abjuratory, apostate, backslider, bolter, breakaway, collaborationist, collaborator, convert, defector, deserter, fifth columnist, mugwump, mugwumpian, mugwumpish, proselyte, quisling, recidivist, recreant, renegade, renegado, renegate, renunciative, renunciatory, repudiative, repudiatory, reversionist, revocatory, runagate, schismatic, seceder, separatist, strikebreaker, tergiversant, tergiversator, traitor, turnabout, turncoat, turntail