Cognitive dissonance has my head in a fuddle. The Dems I know aren't anything like what you keep saying. Agreed that policies don't tend to match the higher "promote the general Welfare" standards, but I suspect there are compounding factors in the stew that are always getting in the way of the advancement of our nobler desires.
One big problem that I have identified is the nettlesome habit of around half of Americans to elect assholes who fight tooth and nail against promotion of the general Welfare, as you point out. This has the effect of the introduction of compromise, which often appears to satisfy what the lawyers tell me; "A good outcome in any dispute is when neither side is happy". Well, that just stinks, and is a perverse attitude towards an ugly reality, but knowing that doesn't change the reality.
I'm going to poke you in the eye just a little bit and say that the position of saying "they are all the same kind of evil bastards" is a lame cop-out that covers no distance in improving the situation, it just makes people who should be friends snarl and snap at each other.
What is the solution? I personally don't think there is one that can be logically and rationally implemented. We humans seem to be seriously flawed, in varying forms and degrees - big problem. We are probably driving towards a major self-extinction event, blindly guided by selfishness and stupidity.
My only dreamer's hope is that more people personally aspiring to better things, and working (however futilely) to make those dream into a molecule of reality, will one day catalyze widespread sanity. (I keep my belief that such a thing is impossible in a little box and do my best to refuse to look at it.)