In what poem does Langston Hughes use the theme of freeing African-Americans?

One good candidate among several for an answer to this question would be Langston Hughes' poem "Let America be America Again." Hughes begins this poem with three stanzas of what might be called boilerplate patriotic verse, attaching to each an ominous coda that signals the way the poem will develop:

(America never was America to me.)
....
(It never was America to me.)
....
(There's never been equality for me,
Nor freedom in this "homeland of the free.")

It is clear that what Hughes is referring to here is discrimination against him as a Black person, but he generalizes the issue into a broader picture of the American underclass in all its variety:

I am the poor white, fooled and pushed apart,
I am the Negro bearing slavery's scars.
I am the red man driven from the land,
I am the immigrant clutching the hope I seek....

I am the farmer, bondsman to the soil.
I am the worker sold to the machine.
I am the Negro, servant to you all.
I am the people, humble, hungry, mean--
Hungry yet today despite the dream.

However, the failure of the "dream" is due not to it being intrinsically faulty, but to it remaining unfulfilled. For its Black population, and more broadly for all of its citizens, America has never existed, since it has never fulfilled its ideals. This means that rather than call America a failure, Hughes declares that it remains to be created:

O, yes,
I say it plain,
America never was America to me,
And yet I swear this oath--
America will be!

Given the ideals on which it is founded, Hughes is saying, nothing with a right to be called "America" will ever exist until both its Black population and all other disadvantaged groups and classes are made free and equal with everyone else. As Hughes wrote in 1943,

And we know it is within our power to help in its further change toward a finer and better democracy....The American Negro believes in democracy. We want to make it real, complete, workable, not only for ourselves--the fifteen million dark ones--but for all Americans all over the land.