US President Donald Trump’s nominee to the Supreme Court appeared all but certain to win approval from the Senate after two key lawmakers endorsed him on Friday.

Senator Susan Collins – a maverick member of Trump’s Republican party – and her Democrat counterpart Joe Manchin said allegations that Brett Kavanaugh had sexually assaulted a fellow teenager nearly four decades ago were unproven.

“I do not believe these charges can fairly prevent Judge Kavanaugh from serving on the court,” Senator Collins told lawmakers in a speech from the floor of the chamber.

Senator Collins backed the embattled pick.

Senator Collins backed the embattled pick.

AP

“The presumption of innocence and fairness do bear on my thinking and I cannot abandon them,” she added, calling Mr Kavanaugh “an exemplary public servant.”

Senator Manchin, who is battling to retain his seat at upcoming midterm polls in a state that voted heavily for Mr Trump in the presidential election, said he had promised constituents to cast his vote “based on the facts before me”. 

Brett Kavanaugh's elevation to the US Supreme Court is all but assured.

Brett Kavanaugh’s elevation to the US Supreme Court is all but assured.

AAP

“Based on all the information I have available to me … I have found Judge Kavanaugh to be a qualified jurist who will follow the Constitution and determine cases based on the legal findings before him,” Senator Manchin said in a statement.

The pair were the last senators to declare their intentions ahead of a final vote which is now expected to take place on Saturday afternoon.

Their declarations of brought the number of senators publicly supporting the 53-year-old judge – who has faced accusations of sexual assault – to 51 in the 100-member chamber.

A final confirmation vote is expected Saturday afternoon.

Mr Kavanaugh’s nomination was left teetering on the edge after university psychology professor Christine Blasey Ford claimed in harrowing testimony last week that he tried to rape her when they were high school students.

Earlier on Friday the Senate voted 51-49 to end debate on Mr Kavanaugh’s nomination, setting up a final showdown on Saturday.

The outcome had remained in doubt, however, after one Republican, Lisa Murkowski, defied her party and voted against moving ahead.

US President Donald Trump nevertheless cheered the result of the cloture vote.

“Very proud of the US Senate for voting ‘YES’ to advance the nomination of Judge Brett Kavanaugh!” the president said on Twitter.

After Senator Collins’s speech, White House press secretary Sarah Sanders took to Twitter to thank her “for standing by your convictions and doing the right thing.”

If he wins confirmation, Mr Kavanaugh – who has faced a bruising process that raised questions over his candor and partisan rhetoric, and his lifestyle as a young man – will seal a conservative majority on the nine-seat high court for years to come.

Trump took the brutal battle to a new stage earlier Friday when he dismissed female anti-Kavanaugh protesters who have cited their own experiences of sexual assault as “elevator screamers.” 

The president claimed billionaire financier George Soros, a frequent target of conservatives, was behind their demonstrations.

President Donald Trump has welcomed the vote.

President Donald Trump has welcomed the vote.

AAP

“The very rude elevator screamers are paid professionals only looking to make Senators look bad. Don’t fall for it!” he tweeted.

Senator Collins appeared to fall in line with Mr Trump’s accusation that outside funding was being pumped into the process, as she slammed the “unprecedented amount of dark money opposing this nomination.”

The confirmation process has gripped Washington and the nation, aggravating already deep political divisions with just weeks to go before mid-term congressional elections.

Among those closely watched is Republican Senator Jeff Flake of Arizona, a frequent Trump critic who is not running for re-election and has expressed concern about Mr Kavanaugh.

On Friday he voted to advance the nominee, then told reporters that barring any dramatic changes, he will vote yes on Mr Kavanaugh’s confirmation.