It is not clear at this point, at least to me, that the woman who was at the party 35 years ago -- call her Ms. X -- is actually the one making the accusation. The reporting is muddled on this, but several stories attribute the initial letter to Anna Eshoo to "someone affiliated with Stanford University," who was relaying information she claimed to have had from Ms. X. When she supposedly received this information is unknown. (A conversation with a college friend 30 years ago? A conversation with an adult friend 15 years ago? A recent conversation in the context of the Kavanaugh nomination?) The accuracy of her recollection is unknown. The extent to which she may be putting the worst construction on ambiguous events to sensationalize a charge is unknown. Whether she is violating a friend's confidence is unknown. Whether Ms. X is willing to substantiate the charges is unknown. The fact that Feinstein sat on the letter for two months suggests that some of these factors may be in play.
At this point, Kavanaugh unequivocally denies the charge and Ms. X has not come forward. Unless Ms. X is the actual author of the letter to Eshoo -- which is unclear -- all we have is hearsay.
Read the linked story from the NYT, which says that three sources confirm that the contents of the letter are being accurately reported. It is curiously worded, if you stop to think about it. At no point does it say the woman who was at the party all those years ago is accusing Kavanaugh of anything. It says "a letter" makes those charges. And it says "the letter" details questionable conduct by Kavanaugh. But it appears to be very carefully written to avoid attributing those charges directly to Ms. X, which would make sense if she was not the author of the letter and if she refuses to come forward.
I can think of 1001 hypothetical scenarios involving three teenagers who had had too much to drink 35 years ago. One of those is that Ms. X has an unpleasant memory of that night but doesn't think the incident should be replayed and sensationalized now. Somewhere along the line, however, she may have shared the experience with someone else "affiliated with Stanford University," and someone else sees a chance to press a hearsay accusation.
And Feinstein is reportedly refusing to show the letter to anybody, which casts more doubt.
But as I said the idea is not to prove a case by the evidence, it’s to scare Senators away from a yea vote.
As to this:
The fact that Feinstein sat on the letter for two months suggests that some of these factors may be in play,I think we conclude that Feinstein was holding the letter until the very end because she thought it was weak evidence to be used ONLY if nothing else worked.
She was probably holding the letter close to her chest because she was afraid that someone might share it with the opposition.
Apparently someone did anyway (ultimately resulting in the letter to Grassley from multiple women attesting to Kavanaugh's good character).