Excellent Research: Obama Citizenship Facts and Obama Birth Certificate Facts
Obama may or may not be Native Born depending on the validity of his paternal grandmother's statement in Kenya, and the sealed hospital records in Mombasa, Kenya, but he is certainly not "natural born" since for that term both his parents must be U.S. citizens (of any category) when he was born. And per statements made in his own campaign's website, and in his books, Obama has admitted his father was a British Subject when Obama was born. Obama's father never at any time was a U.S. citizen. In fact his father was not even an immigrant to the USA nor was he ever even a permanent resident of the USA. Thus since his father was not a U.S. citizen, Obama is NOT a "Natural Born Citizen (NBC)" of the U.S. under our Constitution's framer's intent and Constitutional standards. And thus he cannot serve as our President under our Constitution. Defining "natural born citizen", a dissertation on that subject.
Citizenship Terms Used in the U.S. Constitution - The 5 Terms Defined & Some Legal Reference to Same