The Supreme Court has appeared sceptical of President Donald Trump's executive order limiting birthright citizenship, a sign the justices could strike down a key element of his immigration agenda.
A majority of the court seemed unconvinced the US should stop granting citizenship to children of undocumented immigrants and some temporary US visitors.
The administration has argued that limiting birthright citizenship is necessary to rein in illegal immigration. Opponents argue it would upend more than a century of precedent and unravel a cornerstone of US immigration law.
Trump attended the oral arguments on Wednesday, a rare move by a sitting president that underscored the case's high stakes.
A defeat for the Republican president would mark a second straight setback at the high court, following the decision last month that invalidated his global tariffs. A win would help Trump deliver on his pledge to reshape America's immigration policies.
During more than two hours of arguments, US Solicitor General John Sauer sought to convince the justices that the 14th Amendment - which establishes birthright citizenship and was extended to formerly enslaved people - and subsequent court rulings and laws passed by Congress all mistakenly expanded birthright citizenship.
Chief Justice John Roberts, a key swing vote on the court, questioned Trump's authority to exclude children of undocumented immigrants from receiving US citizenship.
I'm not quite sure how you can get to that big group, Roberts said.
The oral arguments turned on a key clause in the 14th Amendment, which grants citizenship to all people born or naturalised in the US who are subject to the jurisdiction thereof.
Sauer argued that the clause should only apply to the children of foreign diplomats and a few other limited groups. Parents who are in the country illegally when their children are born have allegiance to their home countries and therefore don't fall under the jurisdiction of US law, he said.
But several justices said that interpretation would fundamentally reshape how Americans and people worldwide understand the US birthright citizenship process. Justice Elena Kagan said the administration was seeking to undo a legal tradition of birthright citizenship that dates back to English common law.
What the 14th Amendment did was accept that tradition and not attempt to put any limitations on it. That was the clear rationale, the liberal justice said.
Several justices also pointed to the 1898 Supreme Court ruling, United States v Wong Kim Ark, the landmark decision that upheld birthright citizenship in the case of a child born to Chinese immigrants living in the US. Cecillia Wang, an ACLU attorney representing the plaintiffs in court on Wednesday, used the decision to argue that Trump's executive order should be overturned.
The justices could choose to focus on a 1952 law passed by Congress that codified birthright citizenship and not wade into the larger constitutional debate. The court is expected to issue their decision in June, marking the first major immigration case decided by the court on its merits since Trump started his second term.
Trump's push to end birthright citizenship is one part of his larger immigration crackdown, a longtime goal of many on the right, and something Trump has supported since his first term in office. A victory in this case would help Trump make the case that he is delivering on his campaign pledge to limit illegal immigration, while a loss would be a setback to his immigration agenda, representing a blow to his efforts to expand executive power.
