The prospects for Brett Kavanaugh's confirmation now rest on four swing senators who are carefully reviewing the FBI background report on sexual assault allegations against the Supreme Court nominee, as GOP leaders close ranks behind the embattled judge and one previously undecided Democrat comes out in opposition.

North Dakota Democratic Sen. Heidi Heitkamp, after staying on the fence for weeks, announced Thursday she will vote against Kavanaugh, citing in part his conduct at last week's fiery Hill hearing.

"When considering a lifetime appointment to Supreme Court, we must evaluate the totality of the circumstances and record before us. In addition to the concerns about his past conduct, last Thursday’s hearing called into question Judge Kavanaugh’s current temperament, honesty, and impartiality," she said in a statement, after first announcing her opposition in an interview with WDAY News.

But at least two Republican swing senators indicated they were satisfied with the FBI’s investigation, stirring speculation they could back the embattled judge in the end.

Sen. Susan Collins, R-Maine, who has stayed mum on her Kavanaugh stance, said Thursday that the bureau’s supplemental background probe “appears to be a very thorough investigation.” On Thursday afternoon, she remained in the Capitol building's Sensitive Compartmented Information Facility (SCIF) reading the FBI's report for more than an hour and a half. Senators are taking turns poring over the documents in the SCIF.

And Sen. Jeff Flake, R-Ariz., who originally requested the FBI re-open its investigation into the claims leveled against Kavanaugh by Christine Blasey Ford, agreed with Collins' assessment.

“No new corroborative information came out of it,” Flake said. “Thus far, we’ve see no new credible corroboration—no new corroboration at all.”

But Flake continued to keep the public guessing, returning to view the report again saying he has "more reading" to do. He pulled a surprise last week when he publicly backed Kavanaugh, and then demanded the FBI probe before a final vote.

The other two big undecideds are Republican Sen. Lisa Murkowski, of Alaska, and Democratic Sen. Joe Manchin, of West Virginia.

Murkowski said Thursday she has made "no decision" on Kavanaugh's fate. "I want to be able to read [the report]," Murkowski said, adding that she would "circle back" and return to the Capitol "soon." For his part, Manchin also said he would return again Friday morning to review the report.

“I tried to read as fast as I can," Manchin told Fox News. "There was some more I wasn’t able to get through, so I can finish up tomorrow morning." He added that the report was "helping" him make up his mind.

As Flake indicated, sources say the FBI's supplemental investigation did not turn up any evidence to corroborate the sexual assault claims against Kavanaugh.

Democrats cried foul, describing the probe as incomplete and limited by the White House -- one senator bluntly called it a "bulls---" probe.

But senior GOP senators rallied around Kavanaugh at a press conference Thursday afternoon, vowing to press ahead with a series of votes starting Friday and blasting Democrats for their handling of the process.

"There are two things we know for sure. One thing is the FBI report did not corroborate any of the allegations against Judge Kavanaugh. Second ... is there is no way anything we did would satisfy the Democrats. They’ve always got a reason why the goalpost needs to be moved further down the field," Senate Majority Leader Mitch McConnell, R-Ky., said.

Republicans need at least 51 ‘yea’ votes to confirm Kavanaugh to the high court. A source said the White House is also trying to convince Indiana Democratic Sen. Joe Donnelly to back off his stated opposition. Should the vote tally reach 50-50, Vice President Pence would vote to break the tie to confirm the nominee.

No matter how the swing senators break, the vote will be tight.

Setting the stage, Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, declared earlier Thursday that nothing in the FBI document changed his mind, and that it was time to vote to confirm Kavanaugh.

“I’ve now received a committee staff briefing on the FBI’s supplement to Judge Kavanaugh’s background investigation file," Grassley said Thursday. "There’s nothing in it that we didn’t already know. These uncorroborated accusations have been unequivocally and repeatedly rejected by Judge Kavanaugh, and neither the Judiciary Committee nor the FBI could locate any third parties who can attest to any of the allegations.

“It’s time to vote," Grassley added. "I’ll be voting to confirm Judge Kavanaugh.”

Grassley was the first senator to view the document, which was ordered by the White House after allegations of decades-old sexual misconduct emerged against Kavanaugh after his nomination hearing before Grassley's committee had ended. Other members of the committee and the full Senate were permitted to view the documents in a secure facility later in the day.

In a letter late Thursday, Grassley also refuted the argument, advanced by several top Democrats and commentators, that Kavanaugh had misled senators by inaccurately describing slang terms in his high-school yearbook.

"It turns out that 'boofing' and 'the devil’s triangle' isn’t so scandalous after all and Judge Kavanaugh was being completely honest in his descriptions of both," Grassley wrote. "This is according to multiple new statements that were provided to the Senate Judiciary Committee under penalty of felony from former Georgetown Prep classmates of Judge Kavanaugh’s and friends of a Georgetown Prep graduate who were classmates with Kavanaugh."

He added: "Republicans were interested in the truth. Democrats were interested in politics."

The FBI interviewed nine potential witnesses in search of possible corroboration of Christine Ford's claim the now 53-year-old Kavanaugh forcibly groped her when they were teenagers. Although the report is not expected to be made public, potential witnesses named by Ford previously said they had no knowledge of the party where she claims the attack occurred.

Ford's claims, put forward in a letter that the committee's ranking Democrat, Sen. Dianne Feinstein, D-Calif., produced last month, led to an emotional new hearing a week ago. Ford repeated her claims, and Kavanaugh then tearfully denied them and accused members of the Senate of combing through his past and turning the body's "advice and consent" role into one of "search and destroy."

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Grassley echoed Kavanaugh's charge in urging his colleagues to take a vote, which could occur as early as Saturday.

“Fundamentally, we senators ought to wipe away the muck from all the mudslinging and politics and look at this nomination with clear eyes," Grassley said. "Judge Kavanaugh is one of the most qualified nominees to ever come before the Senate."

Last week, after bipartisan calls from the Senate, President Trump directed the FBI to re-open the background probe of Kavanaugh to investigate the allegations. The White House requested that the investigation last no more than one week.

“This investigation found no hint of misconduct and the same is true of the six prior FBI background investigations conducted during Judge Kavanaugh’s 25 years of public service,” Grassley said.

The White House said Thursday that is “fully confident” that Kavanaugh will eventually be confirmed to the high court.

But attorneys for Ford slammed the investigation for not interviewing the California professor.

“An FBI supplemental background investigation that did not include an interview of Dr. Christine Blasey Ford—nor the witnesses who corroborate her testimony—cannot be called an investigation,” Ford’s legal team said in a statement. “We are profoundly disappointed that after the tremendous sacrifice she made in coming forward, those directing the FBI investigation were not interested in seeking the truth.”

Senate Democratic Leader Chuck Schumer, D-N.Y., also blasted the report, saying he had "fears" the FBI was "too limited" in the scope and time period for the investigation.

Schumer also slammed the fact that only one copy of the FBI's report was provided to Congress. "This is constraining the ability for all senators and the ability of the American public to see the full truth," Schumer said.

But sources told Fox News on Wednesday that a 2009 memorandum of understanding governed the process that resulted in one report being sent to Congress and kept in a safe. The sources added that all 100 senators were authorized to view the document, upon request from the committee and the designated nine committee staffers.

The original request from the Senate was for the FBI to interview four people who were allegedly at the party described by Ford. The White House on Monday asked the FBI to expand that circle. Sources familiar with the FBI’s investigation told Fox News that the bureau interviewed nine people in total, and received a sworn statement from a 10th person.

The source did not reveal the identities of those interviewed, but other sources confirmed to Fox News that the FBI interviewed Kavanaugh friend Mark Judge, who Ford claims was in the room during the alleged incident and Leland Keyser, Ford’s friend who was allegedly at the party. Both witnesses have denied having knowledge of the alleged incident.

Judge, in a letter to the committee last week, said he did “not recall the events described by Dr. Ford in her testimony” nor did he see “Brett act in the manner Dr. Ford describes.”

In a statement to the committee last month, a lawyer for Keyser denied having attended the party.

“Simply put, Ms. Keyser does not know Mr. Kavanaugh and she has no recollection of ever being at a party or gathering where he was present, with, or without, Dr. Ford,” Keyser’s lawyer told the committee.

Sources told Fox News that after the interviews and reviews of statements received, there was no corroborating evidence to back any accusations leveled by Ford.

Despite concerns from Democrats, after reviewing the report, Sen. Lindsey Graham, R-S.C., said he “learned nothing” that he “didn’t already know.”

“He’s probably the most scrutinized guy in America,” Graham said. “I’m more confident than ever ... that the allegations levied against him were not proven to be more reliable. Quite the opposite.”

McConnell, R-Ky., filed cloture on Wednesday evening to end debate on the nomination, setting up a key procedural vote for Friday -- and a possible confirmation vote as early as Saturday.

Fox News' Alex Pappas, Chad Pergram, John Roberts and Mike Emanuel contributed to this report.