partitionist

noun

1. A political partisan who favours geopolitical partitioning.

   “That such antics were really a disguising tactic meant to mask the fact that ministers de Brun and McGuinness are operating a partitionist Stormont government with British civil servants may fool some of the more gullible Shinners, but few were around the north Down and east Belfast fortresses of Education and Health to notice the butcher's apron fluttering outside on millennium day.”

2. (Canada)A political partisan who favours the partition of the province of Quebec with the western, southern portions and Montreal forming a separate province or country from the rest of Quebec.