I watched the interview this morning. The one thing that I noticed is that McCain was "barking" as he made his reply. The words did not quickly come to him and were difficult to find and say.
Barking= a lot of ers, ahs, but , I-I-I type of words used as one speaks.
That's not normal for McCain who is generally pretty straight forward. I'm not saying he didn't believe what he was saying, but clearly he seemed uncomfortable.
Am sure defending Pres Bush was the last thing that McCain wanted to do on the air if I were guessing.