As professor Gatto Trocchi used to say: "the elites have willfully created a culture of disruption and chaos in order to create divisions among the people. So that the elites could defraud the people socially, politically and economically".
But unfortunately, I add these elites are on the verge of their own downfall.
Because the people's awareness is rising at an incredible pace, because they are realizing that their enemy was not their own neighbor.
Or foreigners with a different skin color or a different religion.
The enemy has always been the elites who had created division in the first place.
I'm not sure about the Frankfurt School, but I don't believe that communists or Marxists have had much influence over America.
The (mostly) non-communist labor movement may have helped to bolster liberalism, though. I think the Bolshevik Revolution may have sent a wake up call to Western capitalists into thinking "if we don't stop acting like a bunch of scumbags, then our countries will plunge into revolution too."
That's when capitalists seemed more willing to make deals with labor unions for better wages and working conditions. They appeared willing to compromise rather than risk losing everything they had. For their part, the working classes had no real love of "godless communism" and were content to remain loyal to the capitalist system - as long as they got their fair share. Indeed, after WW2, the American standard of living grew by leaps and bounds - far better than anyone had ever seen previously.
But it was more due to Keynesianism than Marxism, which really never gained any real ideological foothold in America.
As for changes in cultural perceptions and views on racial politics, there were probably several influences at work, some of which had been present even before the Civil War. The Abolitionists and their ideological successors were not Marxists. The industrial capitalists also had their own reasons for opposing slavery, although they seemed somewhat ambivalent regarding racism and the direction US politics took in the Postbellum era (i.e. "separate but equal").
I think the big turning point in US policies on race was around WW2. For one thing, the Nazi policies had gone so far beyond the pale that it caused greater light to be put upon our own racial policies, which became more and more indefensible in the court of public opinion.
Also, due to the impending collapse of the British and French empires and the power vacuum that it was forming in the world, our geopolitical aspirations had to shift in order to keep the post-colonial world within the Western fold. The big fear was that they could fall to communist agitators.
But in places like Africa and South Asia, the US had a big PR problem due to our own history of racism and oppression. In contrast, the Soviets would have been seen as having a 'clean slate' in the eyes of the former colonial subjects, who were never oppressed by the Russians. The US political leadership saw that there was a need for the US to clean up and polish its image in the eyes of the world. They didn't need Marxists to tell them that.
Another aspect of WW2 is that there was a sense that everyone was in it together, along with a push towards national unity, regardless of race, color, or creed. Fact was, we needed People of Color on side, whether serving in the armed forces or working on farms, factories, mines, etc. (It's also a time when women were being called to work in large numbers, and this is viewed as a major impetus in the women's liberation movement.)
I think what hurt America most was not "Marxism" as such, but rather, it was the widespread fear of Marxism and socialism in general. It was this fear that fueled the paranoia which caused the various red scares in America, along with the Cold War and nuclear brinkmanship.
After WW2, America was on top of the world and at the peak of its power. We had a good thing going, but our leaders decided that it wasn't good enough. They wanted more, and that's where they went wrong.
