US President Donald Trump has again taken aim at Democrats accusing them of attempting to ruin Brett Kavanaugh's life.

"The harsh and unfair treatment of Judge Brett Kavanaugh is having an incredible upward impact on voters," he tweeted.

"This great life cannot be ruined by mean and despicable Democrats and totally uncorroborated allegations!"

The harsh and unfair treatment of Judge Brett Kavanaugh is having an incredible upward impact on voters. The PEOPLE get it far better than the politicians. Most importantly, this great life cannot be ruined by mean & despicable Democrats and totally uncorroborated allegations! — Donald J. Trump (@realDonaldTrump) October 4, 2018

Mr Trump's tweet comes as the US Senate Judiciary Committee received an FBI report on sexual misconduct allegations against Supreme Court nominee Brett Kavanaugh.

Senate Judiciary Chairman Chuck Grassley tweeted on Thursday, "Supplemental FBI background file for Judge Kavanaugh has been received."

Grassley is expected to read the FBI report on Thursday morning, followed by his colleagues.

Supplemental FBI background file for Judge Kavanaugh has been received by @senjudiciary Ranking Member Feinstein & I have agreed to alternating EQUAL access for senators to study content from additional background info gathered by non-partisan FBI agents 1/3 — ChuckGrassley (@ChuckGrassley) October 4, 2018

Because the report is confidential, senators will not be allowed to talk about what's in it.

Republicans agreed to ask the FBI for an additional background check on Kavanaugh after his first accuser, Dr Christine Blasey Ford, testified last week that he sexually assaulted her when they were in high school.

Kavanaugh denies the accusation.

Ford's lawyers have said she was not contacted for an interview. But the FBI spoke to a second woman, Deborah Ramirez, who claims Kavanaugh exposed himself to her when they were in college.

Kavanaugh says that accusation is false.

FBI supplement requested Friday sept 28 by bipartisan group of senators w specific scope of current/credible allegations. Dr Ford & Judge Kavanaugh had opportunity to testify under oath b4 public/cmte to tell senators what they know 2/3 — ChuckGrassley (@ChuckGrassley) October 4, 2018

In setting the voting process in motion, Senate Majority Leader Mitch McConnell is likely to call for a final vote over the weekend.

The Kavanaugh allegations have rocked President Donald Trump's effort to put the conservative appeals court judge on the high court.

Although Kavanaugh has denied the allegations of three women, they proved so controversial that Trump directed the FBI to re-open a background investigation.

This FBI material will b handled per 2009 memorandum of understanding/MOU signed btwn Obama WHCounsel & then-SJC Chairman Leahy. Thats latest memorialization of this “loan agreement” of ExecBranch material. Feinstein, Durbin, Schumer, & others were on SJC in 09 & didnt object 3/3 — ChuckGrassley (@ChuckGrassley) October 4, 2018

Sex assault allegation

Christine Blasey Ford, a university professor in California, testified to the Senate Judiciary Committee last week that Kavanaugh sexually assaulted her at a house party in the early 1980s while they were in high school.

Kavanaugh vehemently denied the allegation and further sexual misconduct claims against him from two other women. One of them, Deborah Ramirez, has alleged that he showed her his genitals during a party in college.

As Senate Republicans moved quickly for a vote, Democrats and one key Republican, Jeff Flake, insisted that the FBI be allowed to reopen its background investigation of Kavanaugh. Trump relented, giving the FBI another week to investigate.

0:00 Blasey Ford details alleged Kavanaugh assault. 00:00 / 00:00 Share Share on Twitter

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The controversy over Kavanaugh's nomination comes ahead of November congressional elections in which Trump's Republican Party will battle to keep control of Congress.

In the new background probe, the FBI contacted 10 people and interviewed nine, The New York Times reported. It was not clear why the 10th was not interviewed.

They include three people who Ford says were in the house at the time of the party. One is Mark Judge, a friend of Kavanaugh who the professor says was in the room when Kavanaugh lay on top of her, ground his genitals against her and covered her mouth to keep her from screaming.

Ford and Kavanaugh were not interviewed. The White House said their testimony before the Senate committee was enough, the Times said.

0:00 Trump mocks Ford's testimony. 00:00 / 00:00 Share Share on Twitter

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'Credible investigation'?

Senator Dianne Feinstein of California, the top Democrat on the Judiciary Committee, said the new FBI interview material might be of no value because Kavanaugh, Ford and witnesses identified as corroborators by Ramirez were not interviewed.

Feinstein said those restrictions raised "serious concerns that this is not a credible investigation and begs the question: What other restrictions has the White House placed on the FBI?"

Earlier Wednesday three Republican senators key to Kavanaugh's approval blasted Trump for mocking Ford at a political rally.

0:00 Kavanaugh rejects Blasey Ford's accusations. 00:00 / 00:00 Share Share on Twitter

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Still undecided on backing the conservative judge, Senator Susan Collins weighed in to denounce the president's comments ridiculing Ford as "just plain wrong."

Senator Lisa Murkowski called Trump's speech "wholly inappropriate" and "unacceptable".

Flake, the third Republican swing vote, said there was "no time and no place for remarks like that."

"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," Flake said.

At a Mississippi election rally late Tuesday, Trump ridiculed Ford's accusation that Kavanaugh sexually assaulted her.

The White House denied that Trump had derided the university professor.

'Reprehensible' mockery

Senate Democratic leader Chuck Schumer, called Trump's remarks "reprehensible". He said the president owed Ford an immediate apology.

In her testimony last week Ford said she could not remember some details of the night of the assault, such as how she got to and from the party.

"I had one beer, right?" Trump said before a gathering of supporters, echoing one point Ford did recall.

"'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 added, to cheers from supporters.

With Republicans holding a slim 51-49 majority in the Senate, the loss of any two of their senators would doom Kavanaugh.

None of the three undecided Republicans would say if they had made their minds up one way or another.