US Supreme Court Justice Brett Kavanaugh has sought to put a bruising confirmation battle behind him at a White House ceremony in which US President Donald Trump declared him innocent of sexual misconduct.

"On behalf of our nation, I want to apologise to Brett and the entire Kavanaugh family for the terrible pain and suffering you have been forced to endure," Mr Trump said at the start of a ceremonial swearing-in at the White House.

"Those who step forward to serve our country deserve a fair and dignified evaluation, not a campaign of political and personal destruction based on lies and deception," he said.

Mr Kavanaugh said he was not changed by a "contentious and emotional" battle over his confirmation to the nation’s highest court, which was threatened by sexual misconduct allegations.

He said: "Although the Senate confirmation process tested me as it has tested others, it did not change me."

His confirmation on Saturday, in one of the closest such Senate votes in history, highlighted deep American polarisation ahead of congressional midterm elections on 6 November where Democrats hope to end Republican dominance.

But far from using the aftermath of the battle to try to heal the nation, Mr Trump piled into even fiercer attacks.

Boarding the Marine One helicopter at the White House ahead of a trip to Florida, he branded the sexual assault allegations that threatened to derail Mr Kavanaugh's path to the top court "a hoax" and "all made up, fabricated".

Democrats, Mr Trump said, "tortured him (Kavanaugh) and his family. I thought it was a disgrace".

The president, whose Republicans fear losing at least the lower chamber of Congress in November, angrily predicted that the Mr Kavanaugh row would backfire on Democrats.

"I think a lot of Democrats are going to vote Republican," he said. "I think you're going to see a lot of things happening on 6 November that wouldn't have."

Democrats fought tooth and nail to stop Mr Kavanaugh's candidacy, claiming that the accomplished, conservative-minded judge was not suited to the Supreme Court, which will now tilt decisively to a more Republican-friendly panel.

Then, just as his confirmation seemed inevitable, 11th-hour allegations emerged that Mr Kavanaugh sexually assaulted a 15-year-old girl while he was a 17-year-old student at high school and exposed himself to a female classmate at an alcohol-fuelled dormitory party at Yale University.

No concrete evidence was produced to back up the searing accusations, which Republicans described as a dirty tricks campaign.

After an extra FBI inquiry, which media reports say was drastically curtailed by the White House, also found nothing new, Mr Kavanaugh was finally voted into the coveted post.

Mr Kavanaugh officially took the oath in a more hurried, private procedure on Saturday, but the White House version late yesterday was a chance for the Trump administration to perform the equivalent of a victory lap.

Mr Trump has repeatedly said that putting conservatives on the court - Mr Kavanaugh is his second appointment - was among the top goals of his presidency.

"I've always been told it's the biggest thing a president can do and I can understand that," he said yesterday upon returning from the brief Florida trip.

Mr Trump said the ceremony would be "very special" and a "big night".

He called the Kavanaugh row "a disgraceful situation brought about by people who are evil," and said that the result was "very exciting".

"I'm doing rallies and people are loving that man and loving that choice," he said while in Florida.

In reality, Mr Kavanaugh's confirmation lit a match under existing left-right tension.

The two-vote margin of victory in the Senate made it the closest Supreme Court confirmation vote since 1881 -- and by far the most contentious since Clarence Thomas in 1991. Only one Democrat voted for Mr Trump's nominee.

Mr Kavanaugh's nomination as a replacement for retired justice Anthony Kennedy, long seen as a swing vote, was controversial from the start.

The initial focus of opposition was solely on the conservative views held by the married father of two.

Then came the bombshell testimony from university research psychologist Christine Blasey Ford that Mr Kavanaugh had sexually assaulted her at a gathering when they were in school.

Now that Mr Kavanaugh is confirmed, the nine-justice court, which rules on constitutional questions, is expected to take a more reliably conservative approach.

Since justices serve lifetime appointments, the political consequences are likely to last long beyond Mr Trump's administration.