Still, the emergence of Dr. Blasey’s allegation may have cost Republicans the chance of winning support from any of the red-state Democrats they were hoping to enlist. Senator Claire McCaskill, a Democrat facing a competitive re-election in Missouri, announced Wednesday that she will vote against confirmation of Judge Kavanaugh.

Ms. McCaskill said she was concerned about the accusation against the judge, but based her decision on campaign finance law. “He has revealed his bias against limits on campaign donations, which places him completely out of the mainstream of this nation,” she said in a statement.

Dr. Blasey, 51, a university professor in Northern California who is also known by her married name, Ford, has accused Judge Kavanaugh, 53, of pinning her to a bed, groping her, trying to remove her clothing and covering her mouth to keep her from screaming during a party in the early 1980s when the two were teenagers in Maryland. Judge Kavanaugh has categorically denied the allegation, and the only other person Dr. Blasey said was in the room has also said he does not remember such an assault and had never seen Judge Kavanaugh behave that way.

Another high school friend, Patrick J. Smyth, came forward on Wednesday, saying he was one of two other people Dr. Blasey identified being elsewhere in the house at the time of the alleged assault. In a letter to the Judiciary Committee, Mr. Smyth said he did not remember anything like it.

“I have no knowledge of the party in question; nor do I have any knowledge of the allegations of improper conduct she has leveled against Brett Kavanaugh,” he wrote. He added, “I have never witnessed any improper conduct by Brett Kavanaugh towards women.”

With the conflict over the confirmation and the supercharged allegations taking place so close to the midterms, both sides were deeply anxious about the possible effect and were eager to blame the other. Republicans argued that they have given Dr. Blasey an opportunity to tell her story to the Senate, either in public or in private, while Democrats said the refusal to call in the F.B.I. showed that the president’s party was not really interested in finding the truth.

“This process has been off the rails for a long time now, but the last 24 hours signal a new low,” said Senator Sheldon Whitehouse, Democrat of Rhode Island. He noted that Dr. Blasey has been harassed and forced to leave her own home since bringing her account to the public. “In the face of this, Republicans are jamming her into a ‘take it or leave it’ kangaroo hearing without even the courtesy of proper investigation of her claims. It is a disgrace.”