How do you erode something that is already gone?

This story from TPM noting that there appears to be some conflict between the Bush people and Gonzales. If true, this means that we will see the famous limits of Bush loyalty--and frankly should. But of course, Gonzales is not the problem, the guy who hired him and sent him the list of prosecutors is.

But I found the last quote from the Times piece funny:

TPMmuckraker March 14, 2007 08:53 AM: "“They’re taking it seriously,” said the other of the two Republicans who spoke about the White House’s relationship with Mr. Gonzales. “I think Rove and Bolten believe there is the potential for erosion of the president’s credibility on this issue.”"

Really? The President has credibility left to erode? On what?