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.

"Jurisdiction means allegiance," Sauer said. Citing a previous court opinion, he later argued that "permanent residence and domicile decides (citizenship). That's what the court should be bound by". 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. (BBC)