[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
^^^ CB: And the "first past the post " system will be in place for a long and indefinite period of time in the US.
However, I don't see where the Continental European representational system has been a basis for winning socialism there, cause there isn't socialism there. The "Socialist" parties are equivalent to the DP, ultimately , in the long run.
Back to the US , with only two parties, it might be easier to prevent divisions within the US working class , once it gets its "head" together, and moves in 7 League Boots to dig capitalism's grave. It can just take over one party and make it the Working Class party.