I think Obama thought it would be easy to wipe the floor (as they say) with Romney. After formidable opponents - Hilary and McCain, who Obama truly respected each in their own way - and since he was running away with the polls, I think Obama just thought he could coast through the first debate.
Clearly he learned he can't - and he is a very quick learner.
Obama shines in his capacity to pull it out when he has to.
I wouldn't have believed it could change so dramatically, after 1 debate. That being so, I imagine it could change again after tomorrow. All indications (consumer / economy stats, etc.), are positive for our President!!
I personally think a lot of it has to do with racism. If Obama were not part African-American, he wouldn't be having these challenges.