Anonymous (but credible) 'blogger Philo asks the question: What are non-totalitarian leftism's intellectual foundations? and comes up with the answer that there are none, at least not in the way that libertarianism and conservatism are founded on their respective corpora. Libertarians can claim (among others) Nozick, Richard Epstein, and both Friedmans; conservatives are still heavily influenced by Burke, Chesterton, and in Philo's view, Aristotle and Aquinas. Both groups draw heavily from Hayek. To whom do left-liberals look?
The first thinkers to come to mind are G.A. Cohen and John Rawls. Those familiar with the historical emergence of an American left-liberalism may say "John Dewey" but the thought that today's leftists are inspired by or draw from John Dewey is as mistaken as thinking the typical libertarian is a Nozickian because Nozick was the one libertarian included in your college philosophy course. Cohen is immediately out, because we're considering non-totalitarians. (Thus, also, we disregard Marcuse and Alinsky.) I've known one left-liberal who is really into utilitarianism, but only one; there isn't a movement of people thinking through things using the intellectual framework of Dworkin or Singer.
It's only a bit less wild to establish Rawls as a leftist Hayek. Via The Nation and similar outlets, totalitarians--Friere, Marcuse, Adorno, Cohen--are influential, even if that magazine's readers aren't the would-be Lenins its contributors often are. Rawls is the stuff of philosophy nerds. Regardless: Concerning Rawls, Philo is a bit too dismissive. His approach may be fatally flawed, but not so thin that it wasn't influential. It doesn't at all account for wealth being created, nor for progress, nor for humans not being cicadas entering this world all at once with the passing of each generation. In drawing conclusions from its "maximin principle"--which Philo rightly points out requires the maximization of the welfare of mental patients, which is not what Rawls did--it should lean heavily on the positive social sciences, but what instead came through were Rawls's deeply bizarre prejudices. Lomasky's "Libertarianism at Twin Harvard" is damning, not so much of the Rawlsian methodology, as of Rawls's conclusions--a Rawlsian Rawls with different prejudices may have arrived at a night watchman state (and a Nozickian Nozick with a different emphasis may have been a left-wing radical!) But this makes Rawls more instructive here, not less.
My conjecture: Left-liberalism appeals (even to educated people) despite having little intellectual foundation because it caters to common prejudices, of the sort held by Rawls. Psychologist Jonathan Haidt found that of the six dimensions of moral foundations of political belief he could identify, American left-liberals felt the most strongly (by far) about two: fairness and liberty. This explains why, like Rawls through Lomasky's eyes, they often sound like funhouse-mirror libertarians. Haidt and his collaborators measure naive judgements--prejudices--and not intellectual arguments. In this work, they found further that instinctual/prejudicial fairness to leftists is very different than fairness to libertarians or conservatives: to leftists, fairness is equality, not proportionality. (Why equality and fairness were not made separate dimensions is unclear.) Haidt does not explore sociobiology, but others who have speculate that the concern for material equality is atavistic. In a hunter-gatherer society, if people do not get equal shares, they were probably cheated in the division of spoils. Indeed, one often hears leftists talk of wealth as though it were spoils.
Left-liberals do not call for equal shares. They'll insist up and down that they're not reds and support capitalism, and they're sincere. They can't say where it will end, why they choose this intervention and not that, because their thought is without intellectual foundations. Rawls didn't call for equal shares, either. His methodological mistake and the flaw in egalitarians' moral instincts are the same. His famous hypothetical, in a sophisticated way, causes the question of governmental regulation of wealth or income to be treated as a division-of-spoils problem.
Why care about the question of intellectual foundations? It doesn't detract from the left-liberal position to point out that there are none, aside perhaps from the ghosts of totalitarianisms past. This doesn't in itself render them incorrect issue-by-issue. (There are other reasons for that.) It's important for non-leftists to keep in mind how very different they are from the rest of us. They are not having the same discussion or argument. They are not thinking of the serious questions in the same way. It is not just a difference of opinion.
To someone with different political tendencies, American left-liberals look like half-stepper, non-totalitarian cousins of European social democrats. Some are, and the Web--every man a comparative government researcher!--has given American lefties a more European feel. But even this is superficial. They may always say "well, in [European Country] they do [Caricature of a Policy] and [Usually Fallacious Appeal to Statistics], therefore we should do it too," but the endorsement comes from a lazy consequentialism, not any ethical conviction nor theory of economics nor theory of government. In my experience, they think themselves pragmatists--"I'm for What Works"--and spin themselves (and their intelocutors!) in circles when confronted with complications and more thorough analyses. Ask them why. Point out the moral and material tradeoffs, and ask why they balance things the way they do.
A similarly educated conservative or libertarian would be able to answer. I am not claiming that the average libertarian is a deep thinker like David Friedman or Richard Epstein. Both groups have their own peculiar prejudices--see Haidt's book The Righteous Mind--but for whatever reason, both seek out the wisdom of others in reconciling their perhaps internally conflicting moral instincts and formulating political-philosophical beliefs. Some stop at Ayn Rand, satisfied with false moral clarity. But the step beyond instincts to foundations is there. Why they make this step and lefties don't is, as far as I know, an unanswered question. Maybe it is because leftists are culturally dominant whereas libertarianism and conservatism are countercultural. Maybe it is because the left-liberal prejudices determine public policy positions whereas those of libertarians or conservatives leave more free variables.
The outcome is the same: The left-liberal thinks himself a pragmatist because he is solving, ad hoc, a problem of his own creation: How can I make people free, equal, and better off? Perhaps the best attack plan involves making the problem with this problem nakedly evident whenever possible. There is little moral clarity in a world of tradeoffs.