(CNN) Sen. Susan Collins, who could determine whether Brett Kavanaugh gets a lifetime seat on the Supreme Court, raised serious concerns at a private meeting about the newest allegations of inappropriate behavior against the nominee -- and questioned why the Senate Judiciary Committee had not subpoenaed a close friend of the federal judge.

Multiple sources familiar with the private Wednesday meeting told CNN that Collins appeared unnerved by the latest allegation, citing in particular that it was a sworn statement sent to the panel, which carries with it the possibility of perjury for lying to Congress.

Collins, sources said, did not indicate how she would vote. But her private concerns underscore how Kavanaugh faces a critical test Thursday: He must reassure at least three key GOP senators that he did not act improperly towards women in the 1980s when he testifies after one of his accusers, Christine Blasey Ford. He can't afford to lose more than one GOP senator if all Democrats vote against his nomination.

At the meeting of Republican chairmen, GOP leaders tried to reassure Collins, keenly aware of the critical role she plays.

But it was made clear to senators inside the meeting that the most recent allegation had resonated with the Maine moderate in a way that others up to that point had not.

The sworn statement, Collins told the senators, brought the allegations to a new level and raised concerns that enough wasn't being done to address their veracity. Pointing to the affidavit, which she had printed out, Collins said given the weight of the allegations, it made sense to subpoena Kavanaugh's friend Mark Judge -- an alleged witness to the incidents — and bring him in for testimony.

Multiple senators in the room attempted to reassure Collins that the process in place -- with the Judiciary Committee investigators and staff looking into the multiple allegations, gathering evidence and attempting to interview all involved -- was the most efficient way to handle what had been brought to the forefront.

Senate Majority Leader Mitch McConnell told Collins that the committee staff had begun to reach out to the third accuser, while Senate Majority Whip John Cornyn and others suggested bringing in Judge would do little to shed light on the situation.

Senate Intelligence Chairman Richard Burr, a North Carolina Republican, even brought up his own experience running the Russia probe: He said issuing subpoenas to witnesses could take weeks -- if not months -- and often are not successful. It is unclear how Collins took those comments, and her spokeswoman could not be reached for comment.

Collins, throughout Wednesday, has not spoken at length about the allegation or where she stands in her process of deciding how she would vote on Kavanaugh's nomination. But Thursday's testimony will undoubtedly sway her vote.

"Obviously I take it seriously and believe that it should be investigated by the committee," Collins told reporters of the new allegation. "My understanding is the committee's investigators have already made a request."

Collins has told reporters that she plans to clear her schedule Thursday to watch the hearing, in its entirety, and won't make up her mind until she fully evaluates the testimony.

The latest allegation rocked the already shaky Kavanaugh nomination after a woman -- Julie Swetnick -- wrote a sworn statement to the committee, alleging she witnessed inappropriate behavior by Kavanaugh at more than 10 parties more than three decades ago. Among the allegations: engaging in abusing and physically aggressive behavior toward girls present at the parties.

Kavanaugh has vehemently denied all of the allegations, and many Republicans dismissed it as a political hit led by her attorney, Michael Avenatti.