Speaking on the eve of Judge Kavanaugh's confirmation vote this weekend, Mr. Nadler said that there was evidence that Senate Republicans and the F.B.I. had overseen a "whitewash" investigation of the allegations and that the legitimacy of the Supreme Court was at stake. He sidestepped the issue of impeachment.

"It is not something we are eager to do," Mr. Nadler said in an interview. "But the Senate having failed to do its proper constitutionally mandated job of advise and consent, we are going to have to do something to provide a check and balance, to protect the rule of law and to protect the legitimacy of one of our most important institutions."

Mr. Nadler's comments resembled those of Senate Democrats who pushed aggressively for an F.B.I. investigation into allegations by three women — Christine Blasey Ford, Deborah Ramirez and Julie Swetnick — that Judge Kavanaugh had engaged in sexual assault or misconduct. Democrats said the resulting investigation fell far short of legitimacy. And they have questioned whether Judge Kavanaugh was truthful in his testimony about a number of issues, including his drinking habits, before the Judiciary Committee.