As angry protesters swamped Capitol Hill on the eve of a crunch vote on Friday to advance the nomination of Supreme Court nominee Brett Kavanaugh, the judge made a highly unusual plea to defend his impartiality.The opinion piece by Judge Kavanaugh appeared in The Wall Street Journal hours after Republicans confidently declared that a supplemental week-long FBI investigation found nothing to corroborate sexual assault allegations against President Donald Trump's court pick.Opposition Democrats assailed the FBI probe as an incomplete vetting constrained by a White House determined to push through the lifetime appointment of the conservative 53-year-old judge.Thursday marked a day of high drama and public outrage on Capitol Hill, and furore over Kavanaugh's nomination is dominating the runup to next month's midterm elections in which control of Congress by Trump's Republican Party is at stake.In the op-ed piece Kavanaugh defended his performance during last week's Senate Judiciary Committee hearing at which he denied the allegations, made at the same hearing, of a California university professor.That teacher, Christine Blasey Ford, said he drunkenly groped her and attempted to rape her when they were teenagers attending a party decades ago.In his testimony, Kavanaugh complained about "a calculated and orchestrated political hit fuelled with apparent pent-up anger about President Trump and the 2016 election".But his Journal piece, headlined "I am an independent, impartial judge," appeared aimed squarely at Republicans on the fence who have expressed concerns about his temperament and partisan attacks during the hearing."I know that my tone was sharp, and I said a few things I should not have said, Kavanaugh wrote, arguing he was "forceful and passionate" in denying the allegations against him."I do not decide cases based on personal or policy preferences," he added, saying the country's top court "must never be viewed as a partisan institution".The self-defence came too late for John Paul Stevens, a retired Supreme Court justice who on Thursday said he once believed Kavanaugh to be a fine judge."But I think that his performance during the hearings caused me to change my mind," Stevens said in Florida.'No hint of misconduct'A sea of women - thousands of protesters - marched on Washington on Thursday, bursting into the Hart Senate Office Building to hold loud sit-in protests against the judge.Some held signs calling him a liar and "unfit" to serve. Police arrested 300 protesters."I believe Kavanaugh is part of a Big Old Boys club that is going to protect him no matter what," said Angela Trzepkowski, 55, from Delaware.At a rally of supporters in Minnesota Trump called Kavanaugh "one of the most respected", as his supporters chanted: "We want Kavanaugh."Two of the three Republican lawmakers undecided on the nominee boosted his confirmation chances by signalling they believed the bureau had done a thorough probe."This investigation found no hint of misconduct," Senate Judiciary Committee chairperson Chuck Grassley said in a statement. "There's nothing in it that we didn't already know."Grassley said the full Senate should vote on Saturday on Kavanaugh's nomination - an appointment that could shift the nine-member bench to the right for decades to come.The Senate, where Republicans hold a slim 51-49 majority, is expected to first hold a vote to limit debate on the nominee on Friday at 10:30, one hour after the Senate convenes.All eyes are on the key Republicans who could make or break the confirmation - Jeff Flake of Arizona, Susan Collins of Maine and Lisa Murkowski of Alaska.If all Democrats vote against, Kavanaugh can afford only one Republican defection.Speaking to reporters after reviewing the FBI report, Collins said it "appears to be a very thorough investigation".Flake, a vocal Trump critic who pushed the White House into giving the FBI an additional week to address the accusations against Kavanaugh, signalled his apparent satisfaction, saying the report contained "no additional corroborating information".The top Senate Judiciary Democrat, Dianne Feinstein, said the report appeared insufficient to lay to rest concerns about Kavanaugh."It looks to be a product of an incomplete investigation that was limited perhaps by the White House, I don't know," Feinstein told reporters."We had many fears that this was a very limited process," added Senate Minority Leader Chuck Schumer. "Those fears have been realised."'Partisan histrionics'Republican Mitch McConnell, the Senate majority leader, urged an end to what he called "partisan histrionics" and called for a vote on Kavanaugh's nomination."This process has been ruled by fear and anger and underhanded gamesmanship for too long," McConnell said.Trump nominated Kavanaugh to replace justice Anthony Kennedy, who had been a swing vote on a panel now equally divided between four conservative and four liberal justices.Republicans emerged from studying the FBI report to declare they were even more confident now that no corroboration of the allegations was found.But several Democrats lit into the process, with Senator Ron Wyden branding it a "whitewash" and Senator Mazie Hirono wincing at Republican claims of comprehensiveness."There are dozens of people out there that they could have questioned," Hirono told AFP.Kavanaugh also denies further misconduct claims against him from two other women.In the new background probe, the FBI interviewed nine people, including three Blasey Ford said attended the house party.One is Mark Judge, who the professor says was in the room when Kavanaugh lay on top of her, ground his body against hers and covered her mouth to keep her from screaming.As they mulled Kavanaugh, a Democrat up for re-election in a traditionally Republican state took the courageous step of opposing the nominee."After doing my due diligence and now that the record is apparently closed, I will vote against his confirmation," Senator Heidi Heitkamp said on Thursday.Eleventh-hour pressure campaigns were mounted by constituents in the offices of Collins and Murkowski, where they urged the senators to oppose Kavanaugh.