Listen to a very articulate Obama supporter explain why he chooses to support Obama over Clinton.
I absolutely could not explain it better than this supporter does. It goes a long way toward explaining why Obama supporters are not just Obamaniacs.
If you compare the Obama and Clinton senatorial records, I think you will see an indication that Obama is better at bringing people together than is Clinton. He has demonstrated it more often than has Clinton.
There are numerous videos related to this one to which you can navigate from the above link.
One in particular is a very powerful addition to the first video.