Article II, Section 1, Clause 5: Presidential Qualifications
No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.
This clause establishes three requirements for a person to serve as President.
The first requirement is that the person be a natural born citizen. (“…or a Citizen of the United States, at the time of the Adoption of the Constitution”, may have been of lively interest to, say, Alexander Hamilton, but it is now moot.) The courts have held to a clear and reasonable definition of “natural born citizen.” It is any person who is entitled to citizenship at the moment of birth. Under the 14th Amendment, this includes any person born in the United States and subject to the jurisdiction thereof. It also includes children of U.S. citizens born abroad under most circumstances, since they too are eligible for citizenship from birth. It did not matter than George Romney was born in Mexico, that John McCain was born in the Panama Canal Zone, or if Barack Obama was born in Kenya* — all were eligible for citizenship from birth under U.S. law and therefore natural born citizens.
What are the United States? Obviously this includes the actual states, but to what extent does it include outlying territories? Congress has defined persons in “outlying territories” of the United States (presently just Swains Island and American Samoa) as “nationals” rather than “citizens”, and these persons would not be eligible for the Presidency. On the other hand, persons born in Puerto Rico, Guam, or the U.S. Virgin Islands are citizens, and could therefore aspire to the Presidency.
The exception is persons who are not subject to the jurisdiction of the United States. Since the granting of citizenship to native Americans in 1924, under the Native American Citizenship Act, the only persons living in the United States who are not subject to its jurisdiction are foreign diplomats and soldiers, sailors, or airmen of invading or occupying forces. These are obviously narrow categories (and we fervently hope remain so.) Illegal aliens are subject to U.S. jurisdiction, since they can certainly be arrested, and so are their children born to them in the U.S. — hence, there is no constitutional way to deprive children born to illegal aliens in the U.S. of U.S. citizenship.
What is the reason for this requirement? One of the most important duties of a President is dealing with foreign states. A President born as a citizen of a foreign state, even if he is subsequently naturalized as a U.S. citizen, has at least an appearance of a conflict of interest in dealing with the foreign state of which he was once a citizen, and the Founders wished to eliminate the possibility of such a conflict. One need only look at some of the vitriol directed at John Adams when he was appointed minister to England early in the country’s history. Critics wondered in newspaper editorials whether Adams would be visiting relatives in England. Adams ultimately felt compelled to publish a letter in which he pointed out that none of his ancestors for 150 years had lived anywhere but America. Hence, there is actually a rational basis for what has sometimes been described as an xenophobic clause. (Of course, we do not enact laws merely because they have a rational basis; they should also reflect sound policy and a weighing of all rational arguments for and against them.)
*For whatever it is worth, we believe the evidence is clear and convincing that Barack Obama was born in Honolulu.