I'd go as far as saying the US never had a left to begin with. They have moderates, a hole where conservatives used to be, and an extreme right, right now.
Sounds a big conspiracy theory I know, but well we all know how money heavily influences politics, especially in the us with lobbies and private medias. Don't think we'll see any real left any time soon :(
@Cynic Metalhead: I read that it's partially due private investments. The big corps would sure rather have the republicans in power, but if they can't they still try to push for the least left democrat to be there..