Even IF, and as always that is a massive IF, this were in the slightest bit true, it serves no purpose.
If DiFi had this letter before the hearings, she should have been duty-bound to question him about it and allow him to respond. Instead she went for the cheap, cryptic smear.
If it is alleging a crime that has not had the statute of limitations run, he has a right to confront his accuser publically. DiFi is denying that, so it is baseless.
If it is only alleging bad behavior, again, anonymous sources are notoriously easy to MAKE UP OUT OF WHOLE CLOTH.
Either way, 90% of Senators have made their votes to confirm or not clear. At best, one may change their mind. Personally, I believe that he will be confirmed in a bipartisan vote of 55-45, with three red state Dems attempting to save their electoral chances.