I don't think that prior to the McCarthy era, there were any leftist Americans, other than real-life hard-core commies, like Bolshevists and Trotskyites, among the horribly oppressed proletarians in the industrial boom of the late 1800s and early 1900s.
I won't swear by this, but this is my impression. The Plains Indians and the gold diggers in '49, and the cowboys and the Midwest homesteaders certainly did not think of themselves or of their next-door neighbours, as bleeding-heart liberals, neither did the slave-keepers in the deep South. And nobody gave a hoot about what the black man thought. Most whites in that era were sort of not even consciously but semi-automatically on the opinion that Black people did not even think. And the post-civil-war once-slave population was so uneducated and psychologically and intellectually oppressed, that their inner mental model of the world was most likely the semblance of Jim's as depicted by Mark Twain in "Huckleberry Finn".
The beauty of American history, culture, and mentality, is its satin-white simplicity. Its transparency, its simplicity was cute and adorable when they won WWII to create justice, prosperity and peace for the European nations, but it's not cute or adorable when they drop bombs all over the place just because it annoys them that they don't understand the mentality of the people there. Their pure, simple, crystal-clear mentality can't comprehend the old-world's complex political and diplomatic realities, and thus the only way Americans know how to deal with it is to obliterate such complex nonsense, as they look at it.
Take the last war, the one in Syria. In the beginning the press just ignored it.Then with the rise of ISIS, the press found a clear and easily identifiable "evil" target for the average American's contempt. That new enthusiastic hatred, too, slowly eroded as apathy for the Middle east grew, for the newer news of atrocities and destroyed cities was no longer titillating the mind of the average media reader in America.
It's just too much for the Americans to accept that there is no good and there is no bad people but there are nuances of infinite shades of transition between interests groups and power struggles out there in the world. To the American mind there are basic human rights; of course, in a land where everyone is fat (non only physically) it is not impossible to maintain law and order. In places where 98 percent of the population is literally starving, walks around in rags and is shut out of any kind of "pursuit of happiness", it's harder to understand for an american how come these barbarians don't respect human rights.