Unless the Republicans run someone like Cruz/Santorum (etc.), O'Malley really shouldn't be that hard to beat. He's rather uninspiring, and I don't think he'll come across positively in swing states.
I think he'll be a much more formidable candidate than Joe Biden at least. Not that that is saying very much though, knowing that Biden might very well lose in a landslide margin of possibly more than 10%. I think O'Malley could make the race very competitive though, think Bush/Gore 2000. It would probably be a race that would be decided by a margin of less than 3%, either way. Of course, up until now, O'Malley is more or less totally unknown to the general public.
Biden would win comfortably you reprobate.
Said no one, ever.
Bulls**t. Except of total fringes, it's simply impossible to predict at this stage that someone is definitively going to win or lose badly.