The source is 'The Five Mistakes Clinton Made', by Karen Tumulty.

It may be hard to believe Mark Penn is a fool, but the evidence for it is quite overwhelming

Interestingly, McCain has his own denials about confessing that he himself didn't vote for George W. McCain's flack tells reporters to 'consider the source'. Considering that the source is McCain himself, this is indeed faint praise for one's own mealticket.