The theme of Whitman's poem "I Hear America Singing" is the diversity of American
workers and their contributions to society. The lines that help convey this theme are "Each singing what belongs to him or her and to none else, / The day what
belongs to the day—at night the party of young fellows, robust, friendly, / Singing with open mouths their strong melodious songs." The repetition of the word "singing" emphasizes the idea of the workers' joy in their labor and their collective pride in contributing to the community. Additionally, the repetition of the phrase "what belongs to him or her and to none else" underscores the individuality and uniqueness of each worker's contribution to American society.