Newt Gingrich and Mitt Romney, although they spend a lot of time these days at one another’s throat, appeared on the night of the South Carolina primary to agree on at least one thing: Each believes in “American exceptionalism,” and, they say, Barack Obama does not. Gingrich has already devoted an entire book to the topic, and in an interview with my colleague David Rohde, a top foreign policy adviser to Romney made it clear that American exceptionalism is a theme that Romney intends to stress throughout the campaign.

It’s easy to see that these candidates view their own ideas about American exceptionalism as a strong opportunity to contrast themselves with the incumbent. It’s harder, though, after sifting through the various ways the term is used, to establish what it actually means. Far from being a simple concept that one can easily endorse or reject, American exceptionalism is a loose skein that uneasily unites many different strands of thought, faith and ideology.

Like so much in the discussion of American history, the phrase is often traced to Alexis de Tocqueville’s Democracy in America. But that doesn’t explain much, because when de Tocqueville wrote that “the position of the Americans is therefore quite exceptional,” he was referring primarily to the development of a practical — as opposed to literary or artistic — worldview, stemming from the American landscape and the lack of an aristocracy. More to the point, Gingrich seeks to ground the term in the American Revolution: “The ideals expressed in the Declaration of Independence, and the unique American identity that arose from an American civilization that honored them, form what we call today ‘American Exceptionalism’,” he wrote in A Nation Like No Other, published last year. But that explanation, too, is inadequate; after all, the authors of the Declaration of Independence went out of their way to universalize the values underpinning the American experience (“when, in the course of human events…”), not to cleave that experience off from the rest of the world.

Rather, the faith in the uniqueness of the American experience is best found in its Puritan heritage, the belief that God made a covenant with the founders of America and intended to use American civilization as an example for the rest of the world. In a much-cited speech, Massachusetts Governor John Winthrop referred in 1630 to his colony as “a city on a hill” that could serve as a beacon to the world. But grounding American exceptionalism in religion creates multiple conceptual minefields. For one, the early colonies were not really bastions of liberty; in addition to their slaveholdings, they were, as Gingrich acknowledges, governed like “a theocratic dictatorship.” For another, an appeal to the supernatural puts the idea of American exceptionalism on a similar plane with, say, the Jewish concept of being the Chosen People or the ancient Chinese idea that their country is at the center of the universe — which is to say, there is nothing exceptional about thinking that your civilization is exceptional. Nonetheless, the idea that the United States occupies a privileged and arguably unique place in history is critical to understanding the phrase “American exceptionalism,” from the Manifest Destiny period to the present day.

In the 20th century, American exceptionalism took on a particular meaning in political theory. Typically, it was used to explain why the United States — unlike nearly all developed nations — had never developed a significant working-class political movement. Curiously, although Gingrich and Romney are principally using it in the context of American foreign policy, that usage is of fairly recent vintage. It is also where the meaning of the term is probably the muddiest and does not make as neat a litmus test as Gingrich and Romney seem to want. One can believe that the foundation of America in ideas of liberty and self-governance — rather than in ethnicity or royal domain — makes the United States “exceptional” and yet still be deeply skeptical about America’s use of force abroad. Instead, what Gingrich and Romney appear to be advocating under the name of exceptionalism is either American unilateralism — the idea that the United States has a right and/or obligation to act in the international sphere even if all other countries and multinational institutions don’t join in — or American infallibility — the idea that nothing the United States does in the international arena is ever morally unjustified.