And the proliferation of social media that did not exist in 1991 has added its own heightened effect.

“We’re seeing the same thing years later in a different modern era of communication,” said Tom Korologos, a longtime Washington lobbyist who helped Republican presidents during Supreme Court nominations, although not the battle over Justice Thomas. “A lot is going to depend on how it plays. This is too close to the election for anybody to do anything wrong and take a chance.”

Ron Klain, who was the chief counsel for the Judiciary Committee Democrats at the time of the Thomas hearings, said that to avoid repeating mistakes, the panel should enlist a professional outside lawyer to question both Judge Kavanaugh and Dr. Blasey at a public hearing rather than have senators grill them.

There are a few players still involved from 1991. Senators Charles E. Grassley of Iowa and Orrin G. Hatch of Utah, both Republicans, were on the Judiciary Committee then as now. Back then, Mr. Hatch said Ms. Hill’s account struck him as “too contrived, too slick.” On Monday, he said Dr. Blasey might be “mixed up” and confused Judge Kavanaugh with someone else.

There are some significant differences between the two situations. Justice Thomas was accused of sexually harassing Ms. Hill in the workplace but not physically assaulting her. He was a senior official at two government agencies at the time of the alleged behavior, and she was his subordinate. Judge Kavanaugh is accused of pinning Dr. Blasey to a bed, groping her, trying to remove her clothes and covering her mouth with his hand to muffle her screams while drunk at a party during high school. He was about 17 at the time, and she was about 15.

Neither Ms. Hill nor Dr. Blasey reported the episodes to the authorities at the time, but what has changed since 1991 is the default setting. Whether fairly or not, the burden of proof often seemed to lie with Ms. Hill in 1991, which outraged many who believed her account. Today, in the #MeToo moment, there is more of a presumption that women who come forward with accusations should be given the benefit of the doubt, which worries Judge Kavanaugh’s friends who say he may be smeared unfairly.

Ms. Boxer said she thoroughly believes Dr. Blasey’s account. “The fact is that we know this woman is telling the truth in a case where the woman had everything to lose, tries to remain anonymous, didn’t want the publicity attached to her,” she said.