The Great American Divide

It's obvious that America is becoming increasingly divided, but what is the fundamental cause of the division? Americans have always had differences but those differences did not divide us. From the founding of our nation, Americans have come from different nations, different ethnicities, different religions, different races, and different political ideologies; but we were all united as proud Americans. What has changed? What is the fundamental cause that transformed the traditional American "melting-pot" into this street fight between tribes? What is the real Great American Divide?