[WS:] Bullseye! The only comment that I would add to this excellent point is that it is a product of the US, or more general British, electoral system of the first past the post. In this system any political party that represent minority interest is not politically viable - unlike in the PR system. As long as the first past the post, winner takes all electoral system remains in place in the US, the left has no other alternative but the DP (i.e. a faction within it).
Wojtek