See this transcript of a briefing with reporters. The relevant snippet:

Q Scott, on the National Guard documents, do you have any suspicions about their authenticity?
MR. McCLELLAN: We don’t know whether the documents were fabricated or are authentic. You know, the media has talked to independent experts who have raised questions about the documents. CBS has not disclosed where the documents came from. But, regardless, it does not — the documents do not change the facts. The President met his obligations and was honorably discharged. And the one thing that is clear is the timing and the coordination going on here. There is an orchestrated effort by Democrats and the Kerry campaign to tear down the President because of the direction the polls are moving. And it’s not surprising that we’re seeing the same old recycled attacks. The Democrats are determined to throw the kitchen sink at us, and I suspect this is just the beginning.
Q When you use the word “coordination,” it seems to suggest in a legal sense that the Kerry campaign is illegally coordinating with the 527 —
MR. McCLELLAN: It’s clear. I mean, look at the media reports, they’ve documented the coordinated efforts by Democrats to tear down the President here, because they’re falling behind in the polls. You look at the — The Washington Post had a story about it today, talking about the multi-front effort by the Democratic National Committee, other Democrats. You have outrageous comments being made by Senator Harkin. You have the Democratic National Committee using the term “Operation Fortunate Son.” “Fortunate Son” was the name of a book by an ex-convict that was widely discredited in the 2000 campaign.
Q But what I’m saying is, you’re essentially accusing Mr. Kerry of violating McCain-Feingold, it sounds like.
MR. McCLELLAN: What I’m saying is that the Democrats are seeing the direction the polls are moving and they are now — and they are determined to tear down the President, they are determined to throw the kitchen sink at us.
We’re focused — the President is focused on his agenda for the future and where he wants to lead this country. The Democrats are focused on tearing down the President with the same old recycled attacks, because they’re falling behind.