A high-stakes partisan row has broken out over a confidential FBI report about allegations Brett Kavanaugh sexually abused women three decades ago, with Republicans claiming investigators found "no hint of misconduct" and Democrats accusing the White House of slapping crippling constraints on the probe of the Supreme Court nominee's background.

"Judge Kavanaugh should be confirmed on Saturday," Senate judiciary committee chair Chuck Grassley, a Republican, said at a news conference Thursday. "I feel very good about where this nomination is right now."

He added, in a statement, there is "nothing in [the report] that we didn't already know" and that the FBI found "no hint" of misconduct.

Meanwhile, Republicans moved forward with plans for a key procedural vote on Friday and a final vote on Saturday on confirming the conservative federal appeals judge for a lifetime job on the top U.S. court.

Kavanaugh's prospects of being confirmed by the Senate appeared to be in the hands of four wavering senators. Trump reluctantly ordered the FBI probe under pressure from a handful of Republicans.

Thousands of demonstrators gathered outside the court to protest his nomination, including a loud contingent from Maine calling on Sen. Susan Collins — an undecided Republican — to vote against it. Police arrested about 300 protesters who staged a sit-in on the floor of a Senate office building's atrium.

Protesters opposing Kavanaugh march outside the U.S. Capitol in Washington last Thursday. (Patrick Semansky/Associated Press)

Kavanaugh defended himself again on Thursday in the Wall Street Journal, writing that over the course of his career he has been "hardworking, even-keeled, open-minded, independent and dedicated to the Constitution and the public good."

He said he was "very emotional" as he testified before the Senate judiciary committee and that he knows "my tone was sharp, and I said a few things I should not have said."

"I hope everyone can understand that I was there as a son, husband and dad," he wrote, as the debate about his appointment continued.

Senators speak out

Democratic Sen. Heidi Heitkamp, one of five formerly undecided senators, said on Thursday she would vote against Kavanaugh's nomination.

The North Dakota senator said in a statement that "there are many extremely qualified candidates" for the nation's highest court and she's ready to work with President Donald Trump "to confirm a nominee who is suited for the honour and distinction of serving this lifetime appointment."

Fellow Democrat Joe Manchin said he still had not decided. The key swing vote from West Virginia told reporters — after leaving a secure room where senators were reading the FBI report — that he would return on Friday morning to finish reading it.

Other Democrats spoke out after getting their own briefing.

The judiciary panel's top Democrat, Dianne Feinstein, said it appeared that the White House had "blocked the FBI from doing its job." She said that while Democrats had agreed to limit the probe's scope, "we did not agree that the White House should tie the FBI's hands."

Feinstein complained that agents had not interviewed Kavanaugh or Christine Blasey Ford, who testified that he sexually attacked her in a locked bedroom during a high school gathering in 1982. Feinstein also said lawyers for Deborah Ramirez, who has claimed Kavanaugh exposed himself to her when both were Yale freshmen, had no indication the FBI had reached out to people she had offered for corroboration.

Ford describes how the alleged assault unfolded:

Christine Blasey Ford tells the U.S. Senate judiciary committee how the alleged sexual assault by Supreme Court nominee Brett Kavanaugh unfolded one night in 1982. 1:39

Grassley said the FBI could not "locate any third parties who can attest to any of the allegations," and he said there is "no contemporaneous evidence." He provided no specifics.

Senate minority leader Chuck Schumer, a Democrat, said his party's fears that the "very limited process" laid out for the investigation would restrain the FBI "have been realized."

He also said, "I disagree with Sen. Grassley's statement that there was no hint of misconduct."

Neither side provided any detail about what the report said, constrained by years-old arrangements that require the results of FBI background checks to remain confidential.

'Fishing expedition'

Earlier, White House spokesperson Raj Shah rebuffed Democrats' complaints, saying: "What critics want is a never-ending fishing expedition into high school drinking." He said the FBI reached out to 10 people and interviewed nine, including "several individuals at the request of the Senate, and had a series of followup interviews ... following certain leads."

While the FBI interviews were to focus on sexual assault allegations, Democrats have also questioned Kavanaugh's drinking habits during high school and college, and dishonest comments they say he has made about his background. Kavanaugh has said stories of his bad behaviour while drinking are exaggerated.

Three women have accused him of sexual misconduct in separate incidents in the 1980s. Kavanaugh, 53, now a judge on the powerful District of Columbia Circuit Court of Appeals, has denied the claims.

The White House received the FBI report around 3 a.m. Thursday.

Trump weighed in hours later.

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! —@realDonaldTrump

Democratic Sen. Tammy Duckworth of Illinois, told reporters Thursday that time slots for reading the FBI file are so full that senators are being told they might have to wait until Friday to read it. "They're so swamped," she said.

The report arrived at a Capitol palpably tense over the political stakes of the nomination fight and from aggressive anti-Kavanaugh protesters who have rattled and reportedly harassed senators. Feeding the anxiety was an unusually beefy presence of the U.S. Capitol Police, who were keeping demonstrators and frequently reporters at arm's length by forming wedges around lawmakers walking through corridors.

Barring leaks, it was unclear how much of the FBI report, if any, would be made public. While senators from both sides have expressed support for revealing at least parts of the findings, FBI background checks on nominees are supposed to remain confidential.

With Republicans clinging to a 51-49 Senate majority and five senators — including three Republicans — still publicly undeclared, the conservative jurist's prospects of Senate confirmation could hinge largely on the file's contents.

Undecided senators

The three undecided Republican senators rebuked Trump for mimicking Christine Blasey Ford's responses to questions at last week's dramatic Senate judiciary committee hearing.​ "I had one beer — that's the only thing I remember," Trump stated inaccurately.

Collins told reporters that Trump's lampooning of Ford at a Tuesday night Mississippi campaign rally was "just plain wrong." Sen. Lisa Murkowski called it "wholly inappropriate and in my view unacceptable," and Sen. Jeff Flake said on NBC's Today show that the remarks were "kind of appalling."

Watch Trump criticize Christine Blasey Ford's testimony:

The U.S. president has mocked Brett Kavanaugh's accuser, who testified against the Supreme Court nominee last week 0:44

Those senators, along with Manchin, have yet to declare how they will vote. Collins said Thursday that the FBI appears to have conducted a "very thorough investigation" of the sexual misconduct claims against Kavanaugh.

Partisan divide

Underscoring rising tensions, Democrats suggested that previous FBI background checks of Kavanaugh may have unearthed misconduct by the nominee.

Democrats wrote to Grassley challenging a Tuesday tweet by Republican aides saying prior investigations never found "a whiff of ANY issue — at all — related in any way to inappropriate sexual behaviour or alcohol abuse." Democrats wrote that the Republican tweet contained information that is "not accurate."

Committee Republicans tweeted in response that their prior tweet was "completely truthful" and accused Democrats of "false smears."

Ford, now a California psychology professor, has testified that when the drunken Kavanaugh attacked her, she believed he was trying to rape her.

The FBI interviewed several people, including three who Ford has said attended the 1982 high school gathering in suburban Maryland where she says Kavanaugh's attack occurred, plus another Kavanaugh friend.

With files from Reuters and CBC News