The U.S. Senate voted to advance Brett Kavanaugh's Supreme Court nomination by a margin of 51-49 on Friday morning, with a pair of key Senators indicating they will vote to confirm the nominee in the final Senate vote Saturday.

Kavanaugh's confirmation hearing has been marred by allegations of sexual misconduct and intense protests.

Republican Sen. Susan Collins of Maine addressed the Senate and announced how she would be voting, while Democrat Joe Manchin confirmed his decision shortly after Collins's remarks concluded.

Watch Collins explain why she will vote to confirm Kavanaugh:

Republican Susan Collins explained to the Senate her reasons for supporting Brett Kavanaugh's nomination to the U.S. Supreme Court 0:55

"We've heard a lot of charges and counter-charges about Judge Kavanaugh," Collins said. "But as those who have known him despite the turbulent, bitter fight surrounding his nomination, my fervent hope is that Brett Kavanaugh will work to lessen the divisions in the Supreme Court so that we have far fewer 5-4 decisions and so that public confidence in our judiciary and our highest court is restored."

Kavanaugh appears to have the votes needed to be confirmed following the announcements from Collins and Manchin. The so-called cloture vote on Friday morning was a procedural one to end the debate, and some potential swing-vote senators could conceivably still hold out their support in the final confirmation roll call over the weekend.

Watch Manchin explain his choice amid calls of 'shame' from protesters:

Democrat from West Virginia faced protesters shouting 'shame!' as he explained his vote 1:13

Collins, Manchin and Republican Sen. Jeff Flake of Arizona voted to advance the nomination while Republican Lisa Murkowski of Alaska did not.

"The standard is that a judge must act at all times in a manner that promotes public confidence in the independence, integrity and impartiality of the judiciary and shall avoid impropriety and the appearance of impropriety," Murkowski said in an address to the Senate Friday night.

"After the hearing that we all watched last week, last Thursday, it became clear to me, or was becoming clearer, that that appearance of impropriety has become unavoidable.​"

Watch: Murkowski to vote against Kavanaugh nomination

Republican Sen. Lisa Murkowski explains why she's defying her party and voting against Brett Kavanaugh's Supreme Court nomination. 0:46

Murkowski confirmed that she would vote no again in Saturday's final vote, adding that "we owe it to the people of America to return to a less rancourous confirmation process."

Flake, a member of the judiciary committee who exactly one week earlier pressed for an FBI inquiry into the allegations, said he'll vote to confirm Kavanaugh "unless something big changes."

VP Pence would break a tie

Senate majority leader Mitch McConnell has little room for error with his party's slim 51-49 hold on the Senate. In the event of a 50-50 tie, Vice-President Mike Pence would get a vote to cinch Kavanaugh's nomination.

Donald Trump tweeted Friday after the cloture vote: "Very proud of the U.S. Senate for voting 'YES' to advance the nomination of Judge Brett Kavanaugh!"

The Senate opened with the Republican chairman of the judiciary committee, Chuck Grassley of Iowa, lauding the appellate court judge's deep credentials and lashing out at the "left-wing" groups he said have tried to take down U.S. Trump's nominee as "nothing short of monstrous."

Watch Grassley defend Kavanaugh:

U.S. Senate judiciary committee chair Chuck Grassley says the treatment of Supreme Court nominee Brett Kavanaugh has been 'monstrous.' 1:29

On the other side, Democratic minority leader Chuck Schumer of New York decried the influence of hard-right special interest groups in determining the candidates for Trump's nomination. He also made an implied reference to Merick Garland, the Barack Obama nominee for the later Antonin Scalia's seat who was denied a hearing in 2016, as Senate Republicans argued the vote would take place too close to the November election and the new president should get to fill the seat.

"Let the confirmation process for Judge Kavanaugh be recorded as a sorry epilogue to the brazen theft of Justice Scalia's seat, and the ignominious end of bipartisan co-operation and consultation on the confirmation of Supreme Court justices," said Schumer.

Judge makes last-minute pitch

Kavanaugh, the 53-year-old Federal Court judge made what were in effect closing arguments in a Wall Street Journal op-ed late Thursday. He acknowledged that he became "very emotional" when forcefully denying the allegations at a judiciary committee hearing last week, in which the committee also heard from Dr. Christine Blasey Ford. Ford has accused Kavanagh of forcing her onto a bed during a high school party in 1982 and attempting to take her clothes off as he held his hand over her mouth to stifle her cries for help.

"I said a few things I should not have said," he wrote in the op-ed. But he said he remains the same "hardworking, even-keeled" person he has always been. "Going forward, you can count on me," he wrote in the Wall Street Journal.

Sen. Joe Manchin of West Virginia confirmed Friday that he will support Kavanaugh's nomination. (Mary F. Calvert/Reuters)

Schumer called it the "bitterest partisan testimony I've ever heard from a candidate" seeking Senate confirmation and said there were many other judges conservatives would be happy with on the top court.

McConnell had hoped to press ahead for a Saturday vote after limiting debate, but it is unclear how Republicans will deal with a complication — Republican Sen. Steve Daines has said he's attending his daughter's wedding back home in Montana on Saturday regardless of the Saturday vote.

Murkowski said she will ask to be recorded as "present" during Saturday's confirmation vote to accommodate Daines. Senators often partner like that to allow an absence without affecting the outcome.

In a statement to The Associated Press on Thursday, Daines said two things would happen this weekend: There will be a new Supreme Court justice and Daines will walk his daughter down the aisle.

Daines has supported Kavanaugh throughout the confirmation process.

Trump, Hatch mock protesters

Tensions have been high at the Capitol with opponents of Kavanaugh, including survivors of sexual assault, confronting senators in the halls and holding vigil across the street at the Supreme Court. Supporters of Kavanaugh also turned out.

U.S. Capitol Police said 302 people were arrested on Thursday.

Trump has described the anti-Kavanaugh protesters as "rage-fuelled" and alleged without evidence in a tweet on Friday that some of them were being paid to protest by special interest groups.

A protest rally was held outside the Supreme Court in Washington on Thursday. (Kevin Lamarque/Reuters)

Ahead of Friday's voting, Republicans emerged confident the FBI investigation unearthed no new corroborating details, they said. But a level of uncertainty lingered as Collins and Flake spent hours Thursday poring over confidential FBI documents in the secure basement briefing room.

Grassley said the FBI interviewed 10 people. Six of the witnesses involved Ford's claims, including an attorney for one of them, and four were related to Deborah Ramirez, who has asserted that Kavanaugh exposed himself to her when both were Yale freshmen. Grassley said the FBI concluded "there is no corroboration of the allegations made by Dr. Ford or Ms. Ramirez."

Democrats complained that the investigation, lasting just six days after Trump reluctantly ordered it, was shoddy. They accused the White House of limiting the FBI's leeway.

Those not interviewed in the reopened background investigation included Kavanaugh himself and Ford, who ignited the furor by alleging he had molested her in a locked room at a 1982 high school gathering. Republicans said their testimony last week was sufficient.

Republican Sen. Orrin Hatch of Utah, second from right, and Senate judiciary committee chair Chuck Grassley of Iowa, centre rear, arrive to view the FBI report on Capitol Hill on Thursday. (Alex Brandon/Associated Press)

Utah Sen. Orrin Hatch, a Republican who is weeks away from retirement, was dismissive of Kavanaugh protesters on Thursday.

A few women who identified themselves as sexual assault survivors approached Hatch and asked why he's backing Kavanaugh. Hatch, 84, waved and told them to "grow up" as he entered an elevator surrounded by aides.

As the women yelled at him from the hallway, Hatch smiled and waved.