Probably the biggest problem with Obama is the unknown.
1. We don't know if he keeps his word
2. We don't know much about his policies-this should change as we get closer to November
3. We don't know if he has any skeletons in his closet. Hilliary and McClain do but they already have been exposed so their effect will be minimal. Obama hasn't been under the microscope as long.
Here is a case in point:
http://www.whitehouse.com/NewsCommen...D=110#comments
This is probably bogus and just a money grab but, come Tuesday if this guy passes the test what does that do to Obama's middle of the road support? I would imagine both parties research their opposition's past in the hope of finding some vulnerabilities. The difference between Obama and Hilliary/McClain is that the later too are old news.