The giant who dominates modern American political philosophy is John Rawls. The edifice that he began constructing in the 1950s stands firm today as a philosophical monument for liberal, free-market democracy, and few political theorists have been able to escape from its shadow.
The theory is elegant and powerful. Rawls argues that when we think of how to create a just society, we need to imagine that we are all placed under a "veil of ignorance," where we don't know anything about the various advantages - social or natural - that we are born into. What principles of society would we then agree to? Rawls builds a strong case for two:
Principle 1: "Each person has the same indefeasible claim to a fully adequate scheme of equal basic liberties, which scheme is compatible with the same scheme of liberties for all."
Principle 2: "Social and economic inequalities are to satisfy two conditions: first, they are to be attached to offices and positions open to all under conditions of fair equality of opportunity; and second, they are to be to the greatest benefit of the least-advantaged members of society."
Part of Rawls's appeal lies in the fact that there is something in it for everyone. Liberals and libertarians emphasize Principle 1, which backs egalitarian principles very much like the Bill of Rights. Conservatives see in Principle 2 a nod towards capitalism - inequalities are okay as long as they make everyone better off. Progressives focus on the clause that such inequalities should benefit the least-advantaged members of society. And, egalitarians and meritocrats highlight the importance of equality of opportunity.
Together these pieces provide a philosophical foundation for American liberal democracy, and Rawls built an almost airtight logic to justify it. But, the structure leaks a bit, especially when exposed to practical realities.
One challenge is that there is an inherent asymmetry to his principles. Principle 1 begins with an equal world, and Principle 2 builds on it certain justifiable inequalities. But, these two principles don't suggest how one might start with an unjust world, and then make it increasingly fairer to reach the ideal. There is no "undo" button in the real world for injustices that have already been committed. As a philosopher, of course, Rawls left the political engineering to others. Nevertheless, the theory has a directional bias that explains how inequalities can be generated, but not how they can be undone.
Another practical issue is that in a finite world, there are precious few actions that ensure that the least-advantaged members of society are really better off. Flying a plane pollutes the air everyone breathes. Taking your child out of public school removes the most caring parents from the public system, thereby making it worse off for the other students. Lobbying Congress for certain special interests means someone else's interests are not taken into account. And, under conditions of scarcity, Principle 2 actions are even further constrained.