Increasing suburbanistation from the 1940s onwards led to increasing racial segregation and intensified class divides. The two are linked and the root cause is the same. Particular groups of people would move into the suburbs, and others wouldn’t. This intensified each group’s sense of identity and emphasised divides.
Prior to this, cities were more mixed, although cities have always had areas inhabited primarily by the rich, the poor, the middle class, and particular ethnic groups. However these areas were not isolated from each other in the way that suburbs are separate from the inner city.
What happened in the middle of last century was the flight of middle class white people from the inner cities to the suburbs. There were variety of reasons for this. Racism was certainly part of it and those leaving often wanted a more segregated community and schools.
Then there were also new developments springing up, offering the chance of bigger houses and better lifestyles to those who previously could not afford them. The rise of the automobile made suburban living possible. Further, the government of the time provided subsidies making the option even more available, and these subsidies were also at least partially racially motivated.
The result was racially segregated cities with the white middle class in the suburbs and minorities in the inner cities. Growing up in such communities simply intensifies the divide. Children who go to school with and meet only, or mainly, children of the same race are far more likely to retain predjudices passed onto them by their parents than those attending mixed institutions.
There was also a rapidly developing class divide. The inner cities now consisted of the poor and minorities, with many people falling into both groups. The suburbs were the refuge of the white middle classes. The problems associated with poverty, including crime and addiction, intensified. These contribute to the racial stereotypes still unfortunately prevalent today, black president or not.
Today the racial segregation is gradually lessening. From the 1990s onwards the black middle class began moving to the suburbs as well, albeit more slowly. However if anything the class structure in America is getting ever more rigid. Already a country with little social mobility compared to other developed countries, America's stratified social structure is getting worse. Poverty breeds poverty and children attending poor schools in high crime areas are unlikely to get the qualifications or jobs necessary to socially climb.
While racial issues are still, like it or not, an issue in modern day America, and a serious one, it appears the next challenge will be class. Suburbanisation contributed to an ever-widening gap between the working and middle classes. It might well take equally drastic social and economic change for this to be reversed.