The woman who has accused Supreme Court nominee Brett Kavanaugh of sexual assault all but ruled out appearing at a Senate hearing scheduled for Monday to hear her allegations, insisting that the F.B.I. investigate first.

Speaking through lawyers, Christine Blasey Ford said she would cooperate with the Senate Judiciary Committee and left open the possibility of testifying later about her allegations against Judge Brett M. Kavanaugh, but echoing Senate Democrats, she said an investigation should be “the first step” before she is put “on national television to relive this traumatic and harrowing incident.”

Republicans signaled Tuesday night that they would not negotiate an alternative date and would go ahead with the hearing without her or declare it unnecessary if she refuses to appear, and then possibly move to a vote. They have repeatedly stressed that Monday would be Dr. Blasey’s opportunity to testify, either privately or publicly, and that they planned to move forward with the confirmation process afterward.

The apparent standoff was yet another turn in a high-stakes drama over the president’s second Supreme Court nominee that began Thursday, when the top Democrat on the Judiciary Committee revealed that she had referred the sexual assault allegations to federal investigators. Dr. Blasey, a research psychologist in Northern California, has accused Judge Kavanaugh of sexually assaulting her more than 30 years ago when they were teenagers. A charge that he has categorically denied.