Washington DC - President Donald Trump on Thursday maintained his unprecedented campaign to pressure the Supreme Court, saying it would be a "disgrace" if the independent justices rule against him on US birthright citizenship.

At a White House event, Trump piled pressure on the top court to back his bid to restrict the historic right to citizenship for anyone born on US soil.

Indicating that he thinks the court will instead rule to confirm the constitutional right, Trump said, "It would be a disgrace if the Supreme Court of the United States allows that to happen."

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"This decision by the Supreme Court is a very big one. They'll probably rule against me because they seem to like doing that," Trump said.

It's not clear when the court will issue its ruling on the issue, which lies at the heart of Trump's broader attempt to limit immigration and expel undocumented migrants.

Opponents argue that the Constitution explicitly allows birthright citizenship and that Trump is exceeding his presidential authority.

The Supreme Court's nine justices are dominated by conservatives, but not all vote in favor of Trump's policies all the time.

"It's all up to a couple of people, and I hope they do what's right," Trump said, referring to potential swing votes on the court.

On April 1, Trump added to his pressure on the court by attending a hearing on the birthright case in person – the first sitting president to join the audience in history.

The case stems from an executive order that Trump signed on his return to the White House last year, decreeing that children born to parents in the US illegally or on temporary visas would not automatically become US citizens.

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