The Citizenship Clause of the Fourteenth Amendment says: "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."
Trump and his supporters (including some writers for National Review) may draw support from the phrase “and subject to the jurisdiction thereof.” Some have argued that this language must exclude the children of aliens from citizenship, because aliens owe allegiance to another nation and hence are not under “the jurisdiction” of the United States. But the constitutional text requires only that the children born in the United States fall subject to American jurisdiction, which means that they are governed by American law. Almost all aliens in the United States, even citizens of other nations, still fall within our jurisdiction while they are in our territory: Otherwise they could commit crimes of all sorts without fear of punishment. Other uses of “jurisdiction” in the Constitution, such as in the 13th and 14th Amendments, also refer to the power to govern by law, not national allegiance.
Instead, “subject to the jurisdiction thereof” refers to certain discrete categories of people excluded from citizenship, even though they might be born on U.S. territory. These include the children of diplomats and enemy soldiers at war who are occupying territory. These individuals could be on U.S. territory, but are not subject to U.S. law. A third and obvious category was American Indians. At the time of the 14th Amendment, American Indians were still considered semi-sovereigns who governed themselves with their own laws and made treaties with the United States. But “subject to the jurisdiction thereof” did not grant Congress the power to pick and choose among different ethnic and national groups for citizenship. Instead, the phrase recognized a few narrow exceptions to the general principle of birthright citizenship that has prevailed throughout American history.
The main cost of birthright citizenship is the potential fiscal burden posed by the U.S.-born children of unauthorized immigrants. Unauthorized immigrants are ineligible for virtually all means-tested transfer programs, with the notable exception of the federal child tax credit if they file taxes. But their U.S.-born children are eligible for welfare benefits if their family meets the criteria. However, such children are believed to be less likely than other children to apply for benefits because their families fear that interacting with the government will lead to the parents’ deportation. Eliminating birthright citizenship for unauthorized immigrants’ children therefore may not save as much money as proponents might believe.
Eliminating birthright citizenship is also unlikely to have a large impact on the number of children born here to people who are not legally present in the U.S. The number of “birth tourists” – people who come to the U.S. for the main purpose of giving birth here – is believed to be only about 8,000 a year. The number of children born each year to unauthorized immigrants who live here is much larger, about 300,000 per year, but the evidence from Germany suggests that unauthorized immigrants might actually have more children here if birthright citizenship were eliminated.
Unauthorized immigrants don’t have children here because those children can receive welfare benefits or because they can sponsor their parents for a green card. (The latter can’t even happen until the child is 21 years old, so that would be some impressive patience.) They have children here because they want them to be Americans. The U.S.-born children of unauthorized immigrants symbolize their parents’ American dreams. They came for the opportunity to work hard and in return have a better life, for themselves and especially for their children.