Amy Coney Barrett has been sworn in as a US Supreme Court justice after Republican senators have voted to confirm her appointment, just eight days before the presidential election.

US President Donald Trump revelled in what has been described as one of his signature achievements at a White House ceremony to celebrate Senate confirmation of his third Supreme Court nominee.

The made-for-TV prime-time event on the White House lawn mirrored one a month ago, when Ms Barrett’s nomination was announced, which preceded a coronavirus outbreak among top Republicans including Mr Trump himself.

It came little more than an hour after the Republican-controlled Senate confirmed Barrett to the lifetime appointment on a 52-48 vote, with Democrats unified in opposition.

Her confirmation as successor to liberal Justice Ruth Bader Ginsburg, who died last month, creates a 6-3 conservative majority on the high court.

One Republican, Susan Collins, voted against the confirmation.

“The Barrett family has captured America’s heart. It is highly fitting that Justice Barrett fills the sea of a true pioneer for women, Justice Ruth Bader Ginsburg,” Mr Trump said with a smiling Justice Barrett at his side.

In contrast to the White House event in September, more people wore masks and seats were spread out to ensure social distancing. Several of the Republican senators who voted to confirm Justice Barrett were in attendance, although not Majority Leader Mitch McConnell, who has not been to the White House since August due to concerns about its management of COVID-19.

Conservative Supreme Court Justice Clarence Thomas administered one of the two oaths of office that justices have to take.

In brief remarks, Justice Barrett declared her independence from Mr Trump and the political process even as the president stood behind her.

“The oath that I have solemnly taken tonight means at its core I will do the job without fear or favour and do it independently of the political branches and of my own preferences,” she said.

Amy Coney Barrett is sworn in by Justice Clarence Thomas at the White House. Photo: Getty

Chief Justice John Roberts will administer the separate judicial oath at the court on Tuesday, the court said in a statement.

Mr Trump has said he expects the court to decide the outcome of the election and wants Ms Barrett to participate on any election-related cases that go before the justices.

Justices also take a separate judicial oath, usually at the court itself. The court has not yet announced details for Justice Barrett.

In a controversial move, Mr Trump and senior Republicans pressed the Senate to confirm Justice Barrett before the November 3 presidential election.

Her appointment creates a six-three conservative majority on the top US judicial body. She replaces trailblazing liberal justice Ruth Bader Ginsburg, who died aged 87 just over a month ago.

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On Tuesday, Senate Minority Chuck Schumer said the Republican majority was “lighting its credibility on fire” by proceeding with the vote so close to the election. In 2016, the Republicans blocked then Democratic president Barack Obama’s election-year nominee for months.

“The truth is this nomination is part of a decades-long effort to tilt the judiciary to the far right,” he added.

Republican Majority Leader Mitch McConnell defended Justice Barrett’s nomination.

“We don’t have any doubt, do we, that if the shoe was on the other foot, they’d be confirming,” Mr McConnell said.

“You can’t win them all, and elections have consequences.”

Justice Barrett, a former federal appeals court judge, is Mr Trump’s third selection for the court. He has been able to remake it in dramatic fashion as part of his success in moving the broader federal judiciary to the right since taking office in 2017.

-with AAP