The unknown has undone many a president, and no matter the popularity of an Oval Office occupant, any and all presidents are vulnerable. Of course, one thing that seems to set Obama part from his recent predecessors is his ability to keep an inner calm about tough issues.
Presidents seem to fall into two positive categories: they're one of us, or they're heroes. Both McCain and Obama probably see themselves as potential heroes - presidents who will be looked up to, not presidents everyday people will remark are 'just like me.'