The first two provide mostly money and are further left than the rest -- this is a problem, the donor base must be appeased. Unions provide some money and labor for party activities. They also provide some votes, but a dwindling amount. If blacks even vote 70% Republican, the Democrats will never hold any house of Congress or the White House ever again. Other minorites, particularly Hispanics, lean much more Republican, in part thanks to the Bush brothers. Academia and the campus activist types are a small but vocal part of the party and represent the far left wing, but . . . they are going Green.
Therein lies the problem for the Democrats, if they go to the center, the Greens will peel off the left. The Democrats can't even call for expansion of the welfare state, because the Republican approach to pruning the welfare state is much more popular. Right now, the Democrats are left with one issue -- abortion. That's it. That's why most people who vote Democrat do so, these days. It will be interesting to see what happens with this party because abortion will not be enough for them and I don't see a future issue for them to grab onto.