U.S. Constitution amendment XIV, Section 1:
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
(emphasis added)
The plain meaning is . . . well, very plain! The argument that "subject to the jurisdiction [of the United States]" limits the scope of the amendment to those persons born of mothers who are lawfully present in the nation is ridiculous. The Constitution applies to all persons within its territorial jurisdiction regardless of status. Even in the instance of foreign diplomats, U.S. courts have jurisdiction over such persons. The diplomat's official-acts immunity is not a jurisdictional bar but an affirmative defense that can be asserted before the U.S. court that has subject matter jurisdiction over an alleged crime. However, a formidable appellate advocate, Randy Barnett, has just asserted in the N.Y. Times that very argument, so we will hear more about it.