Based on the statement from whitman's choice of the greatest of all poems (“a
teeming nation of nations"), how is whitman redefining the idea of a poem?

In my opinión this is the first person

This is a complete adjective clause. 

Syllable added to a root word to change its meaning : affixan independent word--example run,walk,book : free morphemeaffix that gives grammatical meaning such as tense or number : inflectiona word part that must attach to a word in order to be usable : bound morphemedegree of comparison formed by adding -er or more to an adjective : comparativedegree of comparison formed by adding -est or most to an adjective : superlativeaffix added to the beginning of a word : prefixaffix added to the end of a word : suffixroot word of lovable : lovetwo roots used together to form one word : combining form