Whitman's break with the past made his poetry a model for the French symbolists? (who in turn influenced the surrealists?) and "modern" poets such as Pound, Eliot?, and Auden?. To get a flavor of this power, consider and read aloud these lines from "Leaves of Grass" (1855) (his most famous poem):
I too lived, Brooklyn of ample hills was mine,
I too walked the streets of Manhattan island, and bathed in the waters around it
I too felt the curious abrupt questionings stir within me,
In the day, among crowds of people, sometimes they came upon me,
In my walks home late at night, or as I lay in my bed, they came upon me,
I too had been struck from the float forever held in solution,
I too had received identity by my body,
That I was, I knew was of my body - and what I should be, I knew I should be of my body.
See the brief essay on Whitman by Galway Kinnell in "Poetry Speaks" (Sourcebooks 2001), which also has on CD what claims to be a live recording of Whitman reading a few lines.