Supreme Court nominee Brett M. Kavanaugh on Capitol Hill on Sept. 5. (Matt McClain/The Washington Post)

WEDNESDAY BROUGHT another sensational allegation in the Brett M. Kavanaugh confirmation saga, on the eve of the Senate Judiciary Committee hearing Thursday in which Mr. Kavanaugh and Christine Blasey Ford, the first woman to accuse him of sexual assault, are due to testify. Even if the latest charges had not emerged, the hearing would not be a fair substitute for an FBI investigation into Mr. Kavanaugh’s behavior with women during high school and college. The new accusation only enhances the case for pausing while professionals gather what facts exist.

Lawyer Michael Avenatti released Wednesday a declaration from Julie Swetnick, a District resident who attended Gaithersburg High School in Montgomery County. The affidavit alleges that at high school parties in the early 1980s, Mr. Kavanaugh displayed “abusive and physically aggressive behavior toward girls, including pressing girls against him without their consent, ‘grinding’ against girls, and attempting to remove or shift girls’ clothing to expose private body parts.” It also alleges that Mr. Kavanaugh was present at gang rapes, including an assault on Ms. Swetnick herself. Mr. Kavanaugh responded that he does not know Ms. Swetnick and that the story is “ridiculous and from the Twilight Zone.” Even so, the declaration says there are other witnesses who can attest to the allegations.

There are still many questions about the Swetnick story, including the identity of those witnesses. Which is exactly the point: Gathering information from people such as Ms. Swetnick and any corroborators is what an FBI probe could do. Senators could wait for investigators to clear the air in an orderly, fact-focused way — whether it supported Mr. Kavanaugh or his accusers — then go back to considering the nominee. If Mr. Kavanaugh is being truthful when he flatly denies all accusations, an FBI probe might turn up accounts that corroborate his story.

But Republicans have resolutely rejected reopening Mr. Kavanaugh’s FBI background investigation. Mr. Kavanaugh’s lawyer had the gall to complain Wednesday that Ms. Ford had not yet turned over lie-detector test records and therapist notes that could corroborate her story, as though the scant evidentiary record were her fault. If Republicans had brought in the FBI as Ms. Ford requested, those files would have been among the first things investigators would have obtained. (Ms. Ford later turned over the lie-detector results and affidavits from people she told about her alleged assault.)

Judiciary Committee Chairman Charles E. Grassley (R-Iowa) said Wednesday that he has about 20 staff members talking to witnesses and gathering evidence, including federal agents on detail from Immigration and Customs Enforcement and the Bureau of Alcohol, Tobacco Firearms and Explosives. Yet both Ms. Ford, who has asked for FBI involvement from the beginning, and Mr. Kavanaugh’s lawyer, who complained about the lack of key evidence, have now said in their own way that the process Republicans chose is unsatisfactory.

Republicans still appear to be aiming for a quick vote on Mr. Kavanaugh, in the Judiciary Committee and on the Senate floor. That would be a travesty of honest deliberation. Thursday’s hearing should not be the end of the inquiry into the behavior of a nominee who has been accused of serious offenses, and who could sit on the Supreme Court for decades.