Had an exchange with Megan McArdle which triggered some thoughts: the issue is whether the US is more united now than in 1950's. McArdle cited the decline of trust in most of our institutions That was in response to my citing the exclusions of Catholics, Jews, blacks, etc. from society and battles over race and the Cold War.
I think really there are different dimensions at play here. In some respects we have a much more national society today; the differences among regions, among segments of society, are much diminished. Strong regional institutions (think department stores or newspapers) have declined, while national institutions like Walmart and Amazon have come to the fore.
But while we're more national in one sense, we're much more specialized in another. In the 1950's there were three TV networks, three news weeklies, etc. So there's much more diversity in other dimensions.
It seems to me people have an intuitive/ideal sense of the United States, of who "we are" and how close-knit we are. Who we include and who we exclude varies, both from person to person and from time to time. Sometimes the decisions are conscious and can be explicitly stated; normally it's more of an unconscious thing. I think in the 1950's probably the average person excluded more people than they would today.