Regionalist parties appeal to the social-liberal streak in their political allies, be it the European Greens or the ELDR. As IM points out, the traditional Communist left tended to be statist and illiberal (seeing both individualism and regionalism as bourgeois or reactionnary). Right at the core of the spat, if you ask me.

guaranteed to evoke a violent reaction from police is to challenge their right to "define the situation." --- David Graeber citing Marc Cooper