A person who supports or advocates defection; someone who believes in or practices abandoning their original allegiance or group.
From 'defection' (Latin 'defectio') plus the suffix '-ist' (one who believes in or practices something). The word is quite rare in actual usage.
This word never became common because 'defector' already covered the meaning, but occasionally you see it in academic writing about Cold War politics—it tried to make defection sound like an ideology rather than just an act of betrayal.
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