To the extent that the President bought all the early press about himself and his messianic tendencies, that would mean that anyone who poses a serious threat to him has to be evil. Thus, Mitt Romney is evil -- and in a way that John McCain and Hillary Clinton were not.
John McCain ultimately wasn't a threat (and, in fact, ran his campaign in a way consistent with Barack Obama's sense that he deserves special treatment). He never really took the gloves off. Nor did Hillary threaten his self-image as someone who deserved to be treated as if he were special (although Bill Clinton did, and relations between them are therefore frosty to this day). What's more, after Iowa, it became clear she was eminently beatable. Even so, he treated her with disdain -- remember "You're likable enough, Hillary"?
In any case, I think the Wall Street Journal's Holman Jenkins hit it on the head last week when he noted that Mitt Romney -- corporate success, self-made millionaire, capitalist -- is precisely the kind of candidate best calculated to drive Barack Obama craaaazy.