WASHINGTON (AFP) - Intensive vetting, lobbying and even a bit of skullduggery are underway as United States President Joe Biden prepares to unveil his nomination for the upcoming vacancy on the US Supreme Court.

Mr Biden, a Democrat, has pledged to nominate the first black woman to sit on the nation's highest court and to reveal his choice before the end of February.

White House Press Secretary Jen Psaki was asked this week about the behind-the-scenes jockeying for the coveted slot on the nine-member bench.

She noted that Mr Biden, 79, is no novice when it comes to placing justices on the Supreme Court.

"As the former chairman of the Judiciary Committee, as the former vice-president, I believe he's probably overseen or been engaged with more Supreme Court nominee processes than anyone in history," she said.

"He is not going to be swayed by public campaigns or public sniping or lobbying efforts," Ms Psaki said.

"He is going to pick an eminently qualified black woman to nominate to the Supreme Court and he has a number of potential choices that he's very excited about."

Mr Biden has said he is focused on four candidates to replace Justice Stephen Breyer, a liberal stalwart who announced last month that he would step down in June at the end of the court's current term.

Among the favourites is Ms Ketanji Brown Jackson, a 51-year-old US Circuit Court judge. The Harvard graduate spent two years as a federal public defender representing indigent defendants.

Another leading candidate is Ms Michelle Childs, a 55-year-old judge in South Carolina who enjoys the support of influential Democratic and Republican lawmakers from her state.

Also believed to be on the shortlist is Ms Leondra Kruger, a 45-year-old California Supreme Court judge who has argued 12 cases for the government before the Supreme Court.

All of the candidates are undergoing extensive background checks by the White House to prevent any unwelcome surprises during their Senate nomination hearings.

Their resumes, their case rulings and their public statements will be given the strictest of scrutiny.

'Open mind'

The US Constitution calls for justices to the Supreme Court to be nominated by the president and confirmed by the Senate.

Confirmation used to be a mere formality but it has become the focus of bitter battles in the Senate in recent years amid heightened partisan tensions.

President Donald Trump's nomination of Mr Brett Kavanaugh in 2018 was nearly derailed by sexual assault allegations against the nominee, charges that he hotly denied.

Democrats have only the slimmest of majorities in the Senate and the White House would like to nominate a justice who could potentially attract a few Republican votes in the chamber.

"I'm not looking to make an ideological choice here," Mr Biden said in a recent interview with NBC. "I'm looking for someone to replace Justice Breyer with the same kind of capacity Judge Breyer had."

The President said he was looking for someone "with an open mind who understands the Constitution".

Ms Childs, the South Carolina judge, benefits from the open backing of the state's powerful Democratic congressman Jim Clyburn.

Mr Clyburn, in exchange for his support during the 2020 Democratic presidential primary, extracted a pledge from Mr Biden that he would nominate a Black woman to the court if there was a vacancy during his term.

"Judge Childs has everything it takes to be a great justice," Mr Clyburn said recently.

Republican Senator Lindsey Graham of South Carolina has also openly voiced his support for Ms Childs.

"She's the one that would get the most Republican votes," Mr Graham told ABC's This Week. "She's somebody, I think, that could bring the Senate together and probably get more than 60 votes.

"Anyone else would be problematic."

Ms Childs' candidacy has come under fire from some on the left, however, because of her past representation of management against employees.

Without mentioning Ms Childs by name, two Democratic members of Congress circulated a letter saying "for far too long, the Supreme Court has been dominated by pro-corporate justices."

Other lobbying efforts have been a bit more underhanded.

According to Politico, a former clerk to Ms Jackson, without her knowledge, anonymously edited the Wikipedia pages of potential rivals Ms Childs and Ms Kruger to "invite liberal scepticism".