Anger is a wonderful emotion - many things have been accomplished by the proper use of anger. But when you create a divide, or if you acerbate a divide that may already be there, the result is that you have to destroy your opponents in order to achieve your goal.
But, if you are angry and you say to everyone - "Look, we have a common problem here and it is a problem that is hurting all of us and if we all pull together we can overcome this problem to the mutual benefit of all of us" then you don't have to destroy your opponents - you win them over to your side. And in that case you have to project the fact that there is ONE America with some problems, not TWO or more Americas ready to slit each others' throats.
Reagan won two huge landslide victories by focusing on the bright side of America - our ingenuity, our capacity to overcome problems. If negativity wins an election the result will be more animosity and pessimism. For the life of me I can't see why anyone would want to be the president of a country he feels like is 50% deeply opposed to him and working to destroy "his" way of thinking.
I would rather have a president who looks at those who disagree with him as potential converts to his/her sunny outlook. The president may never achieve perfect unanimity, but it still beats this "us/them" mentality that destroys rather than builds.
Simply put, Edwards, with his rhetoric, can never lead a united America, no matter how accurate his interpretation of things might or might not be. Ultimately, I believe this will be Clinton's major downfall if she gets the nomination. She was so shrill when hubby was POTUS that she will always be viewed as the most divisive politician in America. But at least she has her experience in the Senate to say, "that was then, this is now." What does Edwards have?