(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.

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