Edward J. Erler from the California State University, San Bernardino and The Claremont Institute for the Study of Statesmanship and Political Philosophy offered this Congressional testimony in 1997. It includes this:

Clearly, the author of the citizenship clause intended to count "foreigners," "aliens," and those born to "ambassadors or foreign ministers" as outside the "jurisdiction of the United States." Senator Howard knew, as his reference to natural law indicates, that the republican basis for citizenship is consent. This is the natural law principle of the Declaration of Independence that proclaims that legitimate governments derive "their just powers from the consent of the governed."

See also "A Dummies Guide to Understanding the Fourteenth Amendment" and "Alien Birthright Citizenship: A Fable That Lives Through Ignorance".