The role of the natural law in politics according to Thomas Aquinas

What is the role of the natural law in the political thought of Thomas Aquinas? Through a reading of the Summa Theologiae and De Regno, I conclude that the natural law plays a modest direct role for Aquinas in politics, but a considerable indirect role. These distinct roles reflect the natural law's two-fold characterization as the dictates of practical reason and as a kind of co-providence with the divine. While the natural law offers the simple guidance of reason, the natural law betokens man's dependence upon God's eternal law, a dependence through which man exercises a co-providence in the world.