"Undermine presidential credibility" is a softball phrasing, for sure, but essentially all it means is "cast doubt on the credibility of the president in war matters". So... everything the administration says about the war is supposed to be held as credible without a second thought? It isn't okay to go, "Hey, maybe they're painting a slightly rosier picture of things to suit their political interests"? You know, especially given how incredibly dishonest they are, and what an incredible slant they put on events and circumstances, that's utterly ridiculous. If anything, the administration has only itself to blame; it's undermined its own credibility by being consistently wrong about how things would go in Iraq. How can anyone, upon hearing another "No, don't worry, everything will be okay, civil war is just a hypothetical", not say "... Didn't you guys say pretty much the same thing about there not being any real chance of a long-term insurgency?"
Why SHOULDN'T we look for reasons doubt their credibility?