The US Supreme Court appeared poised on Wednesday to reject Donald Trump's historic bid to end birthright citizenship following a hearing featuring the extraordinary attendance of the Republican president.

The landmark case is a pillar in Trump's attempts to restrict immigration and his decision to attend oral arguments was unprecedented for a sitting president.

Trump left the hearing following the presentation by his solicitor general, John Sauer, and did not remain for the arguments of American Civil Liberties Union (ACLU) attorney Cecillia Wang, who was defending birthright citizenship.

"We are the only Country in the World STUPID enough to allow 'Birthright' Citizenship!" Trump said in a social media post after returning to the White House.

Trump signed an executive order on the first day of his second term decreeing that children born to parents in the United States illegally or on temporary visas would not automatically become US citizens.

Lower courts blocked the move, ruling that under the Citizenship Clause of the 14th Amendment nearly everyone born on US soil is an American citizen.

Sauer told the justices that "unrestricted birthright citizenship contradicts the practice of the overwhelming majority of modern nations" and "demeans the priceless and profound gift of American citizenship."

"It operates as a powerful pull factor for illegal immigration and rewards illegal aliens who not only violate the immigration laws but also jump in front of those who follow the rules," he said.

It also encourages what Sauer called "birth tourism," in which foreigners come to the United States solely to give birth.

The three liberal justices and several conservatives appeared skeptical of the administration's arguments. Conservatives have a 6-3 supermajority on the court and three justices were appointed by Trump.

'It's the same constitution'

Chief Justice John Roberts, a conservative, asked Sauer how common "birth tourism" is before pointing out that regardless of the numbers it would have "no impact on the legal analysis" of the case.

"We're in a new world now," the solicitor general said, "where eight billion people are one plane ride away from having a child who's a US citizen."

"Well, it's a new world. It's the same constitution," Roberts replied.

The 14th Amendment states that "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States."

It does not apply to those not subject to US jurisdiction -- the children of foreign diplomats, for example -- and Roberts said the government appeared to be seeking to expand the exceptions "to a whole class of illegal aliens," a move he described as "quirky."

Justice Brett Kavanaugh, another conservative, asked Sauer why he was citing the birthright policies of other nations.

"We try to interpret American law with American precedent based on American history," Kavanaugh said. "Why should we be thinking about...other countries? I'm not seeing the relevance as a legal, constitutional interpretive matter."

Justice Neil Gorsuch, another conservative, said that when the 14th Amendment was passed, in 1868, there was no such thing as "illegal" immigration.

"If somebody showed up here in 1868 and established domicile, that was perfectly fine," Gorsuch said. "And so why wouldn't we...come to the conclusion that the fact that someone might be illegal is immaterial?"