US President Donald Trump's two top finalists for the US Supreme Court were summoned to Washington ahead of his prime-time announcement that will set up a confirmation showdown with Senate Democrats.

Trump is expected to select one of two federal appeals court judges, either Neil Gorsuch of Denver or Thomas Hardiman of Pittsburgh, and each was asked to go to Washington today, according to people familiar with the President's decision process. Trump says he will announce his choice this afternoon (overnight NZT).

Either Gorsuch, 49, or Hardiman, 51, would in all likelihood restore the ideological balance that existed before Justice Antonin Scalia's death on February 13, 2016, left a vacancy that has yet to be filled. Each is a Republican appointee with a decade-long record on the federal bench.

The nomination will come amid the controversy over Trump's order restricting travel into the US by people from seven predominantly Muslim countries. Trump fired acting Attorney General Sally Yates after she refused to uphold his executive order. That move angered Democrats, who vowed an all-out fight against his nomination of Jeff Sessions as attorney-general and set the stage for an even bigger clash over the high court vacancy.

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White House press secretary Sean Spicer said polls showed that 70 per cent of voters in November viewed the Supreme Court nomination as an important choice.

"This individual will make those voters and every American very, very proud," he said yesterday.

Republicans have said an announcement this week would provide time for confirmation before the Senate recess scheduled to start on April 9, and potentially let the new justice hear cases during the high court's current nine-month term.

Democrats will be hard-pressed to stop the nomination given the 52-48 advantage Republicans hold in the Senate. Under current rules, Republicans need 60 votes to bring the nomination to the Senate floor. Some Democrats say they will insist Republicans reach that threshold.

"It's really important we have a mainstream nominee, and the way to do that is to require a super-majority vote, as we have now," Senator Debbie Stabenow, a Michigan Democrat, said.

Even so, Republicans could eliminate the filibuster for Supreme Court appointments with a simple majority vote. Senate Republican Leader Mitch McConnell has vowed lawmakers will confirm Trump's nominee.

Judge Neil Gorsuch. Photo / AP
Judge Neil Gorsuch. Photo / AP

Opponents will seek to "paint whoever is nominated in apocalyptic terms," McConnell said on the Senate floor. "It doesn't matter who this president nominates, the left has been rolling out the same tired playbook for decades."

Democrats are still smarting from Senate Republicans' refusal to consider former President Barack Obama's nomination of Judge Merrick Garland to fill the Scalia seat. That nomination would have given the court a majority of Democratic appointees for the first time since 1969.

Gorsuch is a champion of religious liberty known for his crisp, occasionally pointed writing style. He has faulted liberals for an "overweening addiction to the courtroom" and last year hailed Scalia as a "lion of the law".

A study led by Mercer University law professor Jeremy Kidd concluded that Gorsuch is the second-most similar to Scalia of the 21 prospective justices on a list Trump released during the campaign.

Gorsuch is a fourth-generation Coloradan who earned his undergraduate degree at Columbia, his law degree at Harvard and a doctorate in legal philosophy at Oxford.

Judge Thomas Hardiman. Photo / AP
Judge Thomas Hardiman. Photo / AP

Hardiman is a Massachusetts native with working-class roots. His father owned a taxi company, and he drove a cab himself when he was a student. He became the first in his family to graduate from college when he earned his degree from the University of Notre Dame in 1987.

A graduate of Georgetown University's law school, Hardiman would be the only justice on the court currently who didn't earn a law degree from an Ivy League school.

Hardiman has gone further than the Supreme Court in backing gun rights by saying the Constitution's Second Amendment protects the right to carry a handgun in public places. "The need for self-defence naturally exists both outside and inside the home," he wrote.

The nomination could be one of several for Trump. Three other justices - Ruth Bader Ginsburg, Anthony Kennedy and Stephen Breyer - are at least 78.

- Bloomberg