Agreed, unless something changes, I don't see his level of support increasing to a point where he'd actually be able to mobilize a force large enough to begin a true civil war on the occasion of his loss.
That being said, given the virulence and fanaticism of the extremists in his cohort, I can see him advocating violence to the point where domestic terrorism becomes a daily part of American life, much like mass shootings have.
I'm not so much worried about him winning as the damage he'll do when he loses.