He's got a book, and he certainly wants to go out there and promote that book;

link
The relevant clip starts about 8 minutes and 19 seconds in. Scott McClellan trying to discredit Richard Clarke for releasing a tell all a year and a half after he left the administration. It's nice for the irony.
"Why all the sudden, if he had all these grave concerns, did he not raise these sooner?" You have to believe Richard Clarke was a little more forthright with his higher ups than Scott McClellan.
"He is bringing this up in the heat of a presidential campaign." Still true. Senator McCain is reaching out to the people who voted from President Bush, so attacks on Bush damage him.