Question: Which party is more susceptible to splitting?

I would really like to see a discussion of this in the comments. Before the Miers nomination (dammit, I'm off the wagon), I had said that I thought the Republican party was more susceptible to a schism than the Democratic party. Since then, we've seen a fractious couple of weeks in the Republican party. I've personally taken criticism from my left, the middle and from my right, something I didn't think possible. What does everyone think? Is the left or right more susceptible to splits?