Three moderate Republicans who could be pivotal in determining whether the Senate confirms US Supreme Court nominee Brett Kavanaugh criticised President Donald Trump on Wednesday for mocking a woman who has accused the judge of sexual assault.

Jeff Flake, one of three Republicans crucial to Kavanaugh's being approved for the court but seen as undecided, called Trump's ridicule of Christine Blasey Ford in a Mississippi campaign speech "appalling."

"There's no time and no place for remarks like that," Flake said in an interview with NBC News early Wednesday.

"To discuss something this sensitive at a political rally is just not right. I wish he hadn't done it. It's kind of appalling."

Senator Susan Collins, another of the trio of senators yet to come out in support of the conservative jurist, said Trump's comments "were just plain wrong."

Their comments injected fresh doubts over the fate of Kavanaugh, as the Federal Bureau of Investigation reviews the allegations three women have made that the appeals court judge drank heavily and sexually abused women while he was a student in the 1980s.

White House officials back Trump

White House officials have backed Donald Trump’s decision to use a campaign rally to openly mock the woman accusing his Supreme Court nominee of sexual assault.

"The President was stating the facts," White House Press Secretary Sarah Huckabee Sanders told reporters during an official media conference in Washington.

READ MORE US Senate to vote on embattled Brett Kavanaugh after fiery hearing

"The testimony by Dr Ford was compelling but you can’t make this decision based on emotion. It has to be based on fact."

"She’s been treated like a Faberge egg by all of us, beginning with me and the President," Senior Trump Advisor Kellyanne Conway told reporters at the White House.

The President's remarks yesterday signaled a stark turnaround from his show of restraint on the matter over the past week. He has previously called Dr Ford a "very credible witness."

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When asked to explain why the President described her that way only to change his tune, Ms Huckabee Sanders replied: "the President believes that Judge Kavanaugh should be confirmed."

President Trump was cheered on by a crowd of Republican faithful in Mississippi yesterday as he mimicked Dr Ford’s responses to questions at last week’s Senate hearing and joked about her inability to remember details surrounding the night of her alleged attack.

AAP

"'How did you get home?' I don't remember. 'How did you get there?' I don't remember. 'Where was the place?' I don't remember. 'How many years ago was it?' I don't know. I don't know. I don't know. I don't know," he said.

"'What neighbourhood was it in?' I don't know. 'Where's the house?' I don't know. 'Upstairs, downstairs, where was it?' I don't know. But I had one beer. That's the only thing I remember. And a man's life is in tatters. A man's life is shattered."

An FBI investigation into the allegations leveled against Judge Kavanaugh is due to wrap up this week. He has vehemently denied all the accusations.