The legal battle over President Trump's executive order restricting birthright citizenship largely boils down to five words found in the 14th Amendment: "subject to the jurisdiction thereof."The big picture: The court's interpretation of that phrase could have massive implications for who gets to be an American — and therefore enjoy the rights and responsibilities that come with U.S. citizenship.Context: The 14th Amendment states, in part, that "[a]ll 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."The administration argues that the phrase applies to those legally — and not temporarily — in the U.S. The American Civil Liberties Union (ACLU), which is defending the plaintiff in the case, says it applies to most born people on U.S. soil, minus narrow exceptions, such as the children of foreign diplomats.Solicitor General D. John Sauer argued in the administration's petition to the high court that "the jurisdiction thereof" refers to political jurisdiction or allegiance. That "allegiance," he argues, would be "lawful domicile."ACLU attorney Cecillia Wang rejected Sauer's argument during Wednesday's oral arguments before the court, saying that "virtually everyone born on U.S. soil is subject to its jurisdiction and is a citizen," excluding "only those cloaked with a fiction of extraterritoriality."Threat level: Trump's stance effectively undermines more than a century of legal precedent interpreting the 14th Amendment, Axios' Josephine Walker reports.If the justices side with the president, children of H-1B visa holders and children born to parents with temporary protected status are among those who could lose automatic citizenship.What they're saying: Todd Schulte, the president of immigration advocacy group FWD.us, tells Axios that "the arguments put forth by the Trump administration have been out there for a long time and have never, ever, ever gained traction.""In fact, since they've come into office, not a single judge has ruled with them on any of the birthright citizenship cases, not one."The Supreme Court has already established the parameters of jus soli citizenship in the U.S. in a landmark 19th-century decision that the 14th Amendment guaranteed U.S. citizenship to Wong Kim Ark, who was born in the U.S. to Chinese immigrant parents.The bottom line: Justices across the ideological spectrum expressed skepticism about Sauer's argument on Wednesday. Chief Justice John Roberts called the examples Sauer used to support his argument "very quirky."The administration has argued that Trump's executive order is needed to safeguard against modern national security concerns.Roberts acknowledged Wednesday that "it's a new world." But he added, "It's the same Constitution."Go deeper: Supreme Court takes on Trump bid to end birthright citizenship: what to know