The U.S. Supreme Court on April 1 appeared skeptical of President Donald Trump’s executive order excluding the children of illegal immigrants and legal temporary visitors from automatic birthright citizenship.

During the more than two hour hearing, part of which Trump attended in person, the justices seemed concerned about the lawfulness of the order, which is blocked in the lower courts. Some justices suggested it would be difficult to enforce and could strip current citizens of U.S. citizenship.

The justices discussed the significance of the wording of Trump’s Executive Order 14160, which focuses on the 14th Amendment’s citizenship clause. The clause states: “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.”

The executive order states that the amendment has never been interpreted to bestow citizenship universally on everyone born in the United States. According to the order, an individual born in the United States is not “subject to the jurisdiction thereof” if that person’s mother was unlawfully present in the country and the individual’s father was not a U.S. citizen or lawful permanent resident at the time of the person’s birth.

American Civil Liberties Union attorney Cecillia Wang said upholding the order would call into question the status of many current citizens. U.S. Solicitor General D. John Sauer said the order is prospective in effect, meaning it doesn’t affect those born before it was signed.

Justice Elena Kagan told Sauer the federal government was taking a “revisionist” approach to birthright citizenship.

Since United States v. Wong Kim Ark (1898), the federal government has recognized that almost all persons born in the United States are U.S. citizens at birth.

Everyone has thought for a “long, long time” that birthright citizenship was the rule, Kagan said.

Sauer said the citizenship clause was adopted after the Civil War “to grant citizenship to the newly freed slaves and their children whose allegiance to the United States had been established by generations of domicile.”

The legal concept of domicile refers to the place where a person intends to permanently reside.

The clause did not grant citizenship to “the children of temporary visitors or illegal aliens who have no such allegiance” to the United States, Sauer said.

Sauer said the court recognized in Elk v. Wilkins (1884) that “subject to the jurisdiction” means “owing direct and immediate allegiance,” which in turn means the clause “did not extend citizenship to the children of temporary visa holders or illegal aliens.”

Chief Justice John Roberts said the examples Sauer gave to support his interpretation of “subject to the jurisdiction thereof” were “very quirky.”

Roberts said Sauer gave as examples children of ambassadors, children of enemies during an invasion of the country, children on warships, “and then you expand it to a whole class of illegal aliens [who] are here in the country.”

“I’m not quite sure how you can get to that big group from such tiny and sort of idiosyncratic examples,” the chief justice said.

Justice Sonia Sotomayor said the Wong Kim Ark decision quotes early American statesman Daniel Webster, who said it was “well-known that by the public law, a non-citizen, while he is here in the United States, owes obedience to this country’s laws,” regardless of domicile, the taking of an oath of allegiance, or renouncing a former allegiance.