Answer:
Race.
Step-by-step explanation:
Unlike Langston Hughes's poem "I, Too", Walt Whitman's Poem "I Hear America Sing" talks of the people of America as a whole, describing them based on their occupation or profession. While Hughes uses the race description in his poem, saying that "I am the darker brother", he directly addresses the issue of race and skin color. Whitman rather talks of the people as mechanics, carpenter, mason etc. So, the race factor is avoided in Whitman's work.