With tensions boiling over on the eve of a major procedural vote in the Senate on Friday morning on Supreme Court nominee Brett Kavanaugh's confirmation, three of four key moderate swing-vote senators are "trying to get to yes," a GOP source told Fox News.

Those senators -- Arizona Republican Jeff Flake, West Virginia Democrat Joe Manchin, Alaska Republican Lisa Murkowski, and Maine Republican Susan Collins -- have kept their options open throughout the nomination process, but there were outward indications that at least some of them will ultimately support Kavanaugh.

In an unexpected, almost movie-like twist late Thursday, Sen. Steve Daines, R-Mont., announced he's going to attend his daughter's wedding back home in Montana on Saturday -- meaning that even though he strongly supports Kavanaugh, he would be unavailable to immediately vote for his confirmation, which is scheduled to be taken up that evening.

But Fox News has learned that Daines made an unusual late-night phone call Thursday evening to Kavanaugh and personally reassured him that "he has made arrangements to be there to get him across the finish line as needed."

Because Senate Majority Leader Mitch McConnell, R-Ky., started the clock Thursday evening, Republicans cannot technically delay the final confirmation vote unless they secure the consent of all 100 senators. The GOP could, however, hold the session open overnight and allow Daines to vote Sunday if needed.

Daines didn't seem to think his absence would affect Kavanaugh's confirmation, even though Republicans hold a narrow 51-49 majority in the Senate. He told The Associated Press on Thursday that two things are going to happen this weekend: There's going to be a new Supreme Court justice, and Daines is going to walk his daughter down the aisle.

The development was the latest in a day marked by uncertainty and drama. For several hours, senators from both parties filed in and out of the Capitol Building's Sensitive Compartmented Information Facility (SCIF), where they pored over the FBI's recently completed supplemental background report on Kavanaugh.

Collins said Thursday that the bureau’s supplemental background probe “appears to be a very thorough investigation.” On Thursday afternoon, however, she remained in the SCIF for more than an hour and a half, causing some consternation among Republicans.

“All of that time, she still doesn’t know?” one source asked Fox News.

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

"It turns out that 'boofing' and 'the devil’s triangle' isn’t so scandalous after all." — Senate Judiciary Committee Republicans

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

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

The other two big undecideds are Murkowski and Manchin. 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.

Sources tell Fox News the White House is also continuing to try to persuade Indiana Democratic Sen. Joe Donnelly to reverse his announced opposition to Kavanaugh. While Donnelly, who hails from a red state, said in September he could not support Kavanaugh "under these circumstances," some of the factors he cited at the time -- such as the lack of an FBI probe -- have since been addressed.

Top Democrats, though, minced no words about the FBI's report, saying the bureau's inquiry should not have been restricted to one week. President Trump has said the FBI had the authority to interview "whoever" they wanted, but Democrats also alleged that the administration had meddled in the investigation.

The time limit, Flake and other Republicans said, was necessary to avoid bogging down Kavanaugh's nomination with a never-ending probe into the accusations, which all related to alleged events more than three decades ago. None of the claims against Kavanaugh appear to have had first-hand corroboration, and the credibility of some of his accusers has come under question in recent days.

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JULIE SWETNICK'S EX-BOYFRIEND SAYS SHE 'EXAGGERATED EVERYTHING,' THREATENED TO KILL HIS UNBORN CHILD

"Well, that report -- if that's an investigation, it's a bull---- investigation," Sen. Bob Menendez, D-N.J., told a man as he walked through the Capitol complex on Thursday. "The reality is, that was not a full and thorough investigation."

North Dakota Sen. Heidi Heitkamp, a red-state Democrat, announced her opposition to Kavanaugh Thursday, citing the FBI report and what she called doubts surrounding his character. Heitkamp is facing long odds in her November re-election bid, apparently owing in part to her reluctance to support Kavanaugh. Fox News' polling shows Republican challenger Kevin Cramer now leading her by 12 points (53-41 percent). Last month, he was up by only 4 points (48-44 percent).

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Meanwhile, on Thursday, Judiciary Committee Chairman Chuck Grassley, R-Iowa, cited new statements that appeared to validate the innocuous explanations Kavanaugh offered last week under oath for several terms that he used in his high-school yearbook. Democrats questioned Kavanaugh at length about some of the phrases Kavanaugh had included, prompting top Republicans, including Sen. Lindsey Graham, R-S.C., to denounce the proceedings as an embarrassing "sham."

"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 in a statement released by the Judiciary Committee. "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 and friends of a Georgetown Prep graduate who were classmates with Kavanaugh."

Late Thursday, Grassley also ripped into Ford's attorneys for demanding that the FBI first interview their client before they turned over her therapist notes, which Ford has extensively relied on as a kind of corroborating evidence for her claims. The notes reportedly indicate that Ford claimed there were four boys in the room during her alleged attack -- which she has said was a transcription error by ther therapist -- and provide some description of her attacker, without naming Kavanaugh.

"Your response on behalf of your client is a non-sequitur," Grassley wrote in a letter. "It’s not even clear to me what purpose turning over these materials to the FBI would accomplish. The FBI would simply turn over that evidence to the Senate. That is precisely the outcome I seek with this request."

Furthermore, Grassley added, "The U.S. Senate doesn’t control the FBI. If you have an objection to how the FBI conducts its investigations, take it up with [FBI] Director [Christopher] Wray."

As senators shuffled in and out of the SCIF, the scene was tense both inside and outside the Capitol building. Thousands of liberal protesters, fired up by Massachusetts Democratic Sen. Elizabeth Warren, marched up Capitol Hill on Thursday to demonstrate. And Wednesday evening, House Minority Leader Nancy Pelosi, speaking at a NARAL Pro-Choice America event in front of the Supreme Court, seemingly compared Kavanaugh to North Korean dictator Kim Jong Un.

According to the U.S. Capitol Police, 302 people were charged Thursday for unlawfulling demonstrating in Capitol buildings. A total of 293 were arrested in the atrium of the Hart Senate Office Building for unlawfully assembling and obstructing, and nine were arrested in the Dirksen Senate Office Building later in the afternoon.

In an op-ed published in The Wall Street Journal Thursday evening, Kavanaugh addressed complaints from Democratic law professors and liberal luminaries -- including retired Associate Justice John Paul Stevens -- that he was too belligerent during last Thursday's testimony before the Judiciary Committee.

Observers said the contrite piece was likely written to appeal to swing-vote senators still weighing his fate late Thursday.

At one point, the judge responded to a question about his past drinking habits from Sen. Amy Klobuchar, D-Minn., by repeatedly asking if she'd ever had alcohol-related blackouts. Later in the hearing, he apologized for the remark.

"I was subjected to wrongful and sometimes vicious allegations," Kavanaugh wrote. My time in high school and college, more than 30 years ago, has been ridiculously distorted. My wife and daughters have faced vile and violent threats.

"I was very emotional last Thursday, more so than I have ever been," he added. "I might have been too emotional at times. ... I hope everyone can understand that I was there as a son, husband and dad. I testified with five people foremost in my mind: my mom, my dad, my wife, and most of all my daughters."

Kavanaugh, who currently sits on the D.C. Circuit Court of Appeals, concluded: "Going forward, you can count on me to be the same kind of judge and person I have been for my entire 28-year legal career: hardworking, even-keeled, open-minded, independent and dedicated to the Constitution and the public good."

Fox News' Alex Pappas, Chad Pergram, Brooke Singman and Peter Doocy contributed to this report.