Whitman's "I Sing of America" is the poem in which Hughes' work provides a wonderful complement.  In Whitman's work, there is an extolling of the diverse nature of American society.  Its heterogeneity is on display for Whitman and the reader.  Whitman argues that this becomes the fabric for democracy and its endurance.  When Hughes writes his poem, it is more of a response to the difference between the theoretical perspective of diversity and the realistic condition of what it means to be "different" in American Society at the time.  The narrative of someone isolated, silences, and marginalized from the dinner table, yet growing in resolve to be heard, acknowledged, and validated provides excellent discussion on the nature of the gap between the promises of America and its reality.

If you look at this poem, you will very quickly see which of the poems it is referring to.  After all, the Hughes poem starts "I, too, sing America."  That means that it is very clearly a response to Walt Whitman's famous poem "I Hear America Singing."

In Whitman's poem, the speaker talks about all the people he sees and hears.  He is saying that all of those people are what make up America.

What Hughes is doing in his response is saying that Whitman's was an incomplete view of America.  He is emphasizing that there is a black part of America that gets hidden and ignored.