At best, this news has no impact on him. The problem is, he needs to dramatically shake up the race and make up a significant deficit.
He can't do that if the focus is on Dan Rather's forged docs.
I think it will take about a week before the picture clears up enough.
One good thing about the guard issue is that--even assuming the worst--it simply isn't lurid or sensational enough to have legs sufficiently strong to carry it past about a week in the news cycle. Most people who view it objectively are likely to conclude it is petty and will quickly become bored with it.