• (n.) An advocate of disunion, specifically, of disunion of the United States.
Dis·un'ion·ist noun An advocate of disunion, specifically, of disunion of the United States. 
disunionist 1. Someone who advocates or causes disunion. 2. In U.S. History, a secessionist during the period of the U.S. Civil War.
a person who advocates or causes disunion. · a secessionist during the period of the Civil War.