After discussing the sources of the American political tradition, I will turn to the Catholic "connection" and suggest that the contribution of Catholicism to any of the sources has been minimal, and the Declaration of Independence in particular betrays no direct Catholic influence. Yet, Catholicism shares the Declaration's affirmation of natural law, which means, ironically, that Catholicism is closer to the Declaration than Protestantism (which has generally rejected natural law theory). Thus, Catholicism and Americanism have natural law in common, as John Courtney Murray pointed out. But Catholic natural law is traditionally derived from St. Thomas Aquinas who has a different version of natural law than the Declaration of Independence and John Locke. The Thomistic-Catholic version of natural law emphasizes the perfection of the rational creature through virtue and favors constitutional monarchy, while the LockeanAmerican version emphasizes inalienable natural rights and favors constitutional democracy or republicanism.
The new twist in the relation is the incorporation of natural rights into the Catholic natural law teaching over the last century. This development has been accompanied by the scholarly claim that natural rights actually began in the Middle Ages and were a discovery of Catholicism rather than of the Enlightenment. If this is so, then Catholicism anticipated natural rights and may now be seen as compatible with the Declaration, even though it did not contribute historically to the Declaration and was often hostile to the rights of man in the eighteenth century. According to this revisionist view, Catholicism provides the authentic grounding of the natural rights principles proclaimed in the Declaration of Independence and may further imply, as John Courtney Murray was wont to argue, that only Catholic natural law could save America.
In judging these various claims, I will argue that the Catholic revisionists of the twentieth century have exaggerated the similarities between Catholicism and the Anglo-American natural rights tradition: Catholic natural law is not primarily about natural rights and government by consent of the people; but about the natural ends of man as a rational creature and the use of prudence to apply these ends to politics. The prudent application of natural law, however, does not automatically point to democracy and natural rights as the best regime, because the natural ends of man are often best realized in a mixed regime that includes hierarchical or undemocratic elements. From this perspective, constitutional monarchy is the first choice and constitutional democracy a second choice.
The conclusion I shall draw is that Catholics can endorse the American version of natural law expressed in the Declaration of Independence as a partial version of Thomistic natural law. They can also embrace the American constitutional order as a decent second choice compared to more hierarchical regimes and support it with a high degree of loyalty and patriotism. But prudence also counsels Catholics to be wary of the abstract rights flowing from the Declaration of Independence (especially the sweeping right to pursue happiness as one sees fit) and to nurture instead those strands of the American political tradition outside of natural rights principles which direct citizens to a notion of "ordered liberty" that contains the whole truth about God and man.Yes it's true that Protestantism has generally rejected the natural law. But not all orthodox Protestants, indeed, not all notable orthodox Protestants have been "fideists" like Francis Schaeffer was. See this article by J. Daryl Charles on reformed natural law.