If you can't deny the information, then attack and seek to destroy the credibility of the messenger, the bearer of the information. And in this case, its change the subject from the truth of the information to the truth of the documents. This is your basic fogging machine, which is set up to cloud the issue, to obscure the truth.
Since when is the onus on anyone to believe Dan Rather and 60 Minutes? You have an accusation, back it up. Or shut up. Somebody - let's say it's me - types a document in MS Word to the effect that Dan Rather is full of crap. Ok Dan, answer the question. I have the documents to "prove" it.