Some years ago political scientist Ruy Teixeira predicted a demographic surge for Democrats which did not materialize. I respect the fact he admitted he was wrong and has moved on. We don't often agree about policy choices but I find much of his analysis first rate.
A Teixeira article in The Atlantic reprinted by msn.com documents the steps by which the Democrats ceded their long-time position as defender of the working class to Republicans. It's a story semi-familiar to those of us who follow politics, but Teixeira lays it out succinctly.
In a de facto two party political system, assembling a potentially majority coalition is the only meaningful goal. Teixeira doesn't make his own views explicit but I infer he believes ceding the working class vote to the GOP was a Dem mistake, perhaps a grievous one.