The seminal Supreme Court case that deals with the issue of birthright citizenship is United States v. Wong Kim Ark, from 1898. In Wong Kim Ark the Court held that a child of transient Chinese nationals, but born in the United States, was automatically a citizen at the time of his birth. The Court wrote, in part:
The Fourteenth Amendment affirms the ancient and fundamental rule of citizenship by birth within the territory, in the allegiance and under the protection of the country, including all children here born of resident aliens, with the exceptions or qualifications (as old as the rule itself) of children of foreign sovereigns or their ministers, or born on foreign public ships, or of enemies within and during a hostile occupation of part of our territory . . . Every citizen or subject of another country, while domiciled here, is within the allegiance and the protection, and consequently subject to the jurisdiction, of the United States . . .
And that Wong Kim “. . . if born in the country, is as much a citizen as the natural-born child of a citizen.”
The Wong Kim Ark case appeared at first glance to settle the issue, but opponents of birthright citizenship noted that the parents in the case were in the United States legally, and therefore had applied for and accepted some form of allegiance to and protection from the United States for the tenure of their stay in the country.
The question was answered, however, in a different form as recently as 1982, with the case of Plyler v. Doe, where the Court struck down a Texas state law prohibiting the use of state funds for the education of the children of undocumented immigrants. Essentially, the Court held Texas’ use of a distinction between legal and illegal resident aliens in the process of determining who was “subject to the jurisdiction” of the United States was erroneous, writing that “no plausible distinction with respect to Fourteenth Amendment ‘jurisdiction’ can be drawn between resident immigrants whose entry into the United States was lawful, and resident immigrants whose entry was unlawful.”
So, the current consensus in the case law is on the side of proponents of birthright citizenship. If that were the end of the story, then any executive order issued by President Trump would be ruled unconstitutional insofar as it did away with the practice.
However, this is not the end of the story. Case law is both binding and non-binding on the Supreme Court, under an oftentimes confusing and contradictory tradition of stare decisis (meaning literally, “to stand by things decided”). Basically, the Court follows precedent created by past decisions until it decides that those past decisions were wrong. Different justices have varying conceptions of how serious a deviation from the correct interpretation of past case law must be before that precedent can be deemed sufficiently “wrong” and overruled.
Justice Thomas, for instance, holds no love for stare decisis at all, and is of the persuasion that if a previous case is wrongly decided it should be overruled, no matter the political disruption it may cause, nor the weight of the legal framework being overturned. Other, more institutionally-minded and tradition-inclined justices exist somewhere on a scale ranging from only overturning precedent when there is unanimity and the precedent is obviously contrary to justice, to overturning precedent more frequently, when doing so will not overwhelmingly disrupt the political process.
Therefore, current Supreme Court precedent that the 14th amendment confers automatic citizenship as a birthright to all persons born in the territory of the United States can be overturned. If the original public meaning of the citizenship clause of the 14th amendment was contrary to the formulation found in modern Supreme Court jurisprudence, then a Court that now leans conservative for the first time in over 70 years might make the decision to overrule past cases and effectively revoke the constitutional guarantee of citizenship by birthright.