What are Whitman's leading ideas in the 1856 letter to Emerson?
I think that one of the most leading ideas that emerges from Whitman's letter is his passionate belief in Emerson's notion of a defining American literary tradition emerging. Whitman believes in this and wishes to share it with Emerson, one he calls "Master." Consider one of the closing parts to this letter:
Of course, we shall have a national character, an identity. As it ought to be, and as soon as it ought to be, it will be. That, with much else, takes care of itself, is a result, and the cause of greater results. ....Such character is the brain and spine to all, including literature, including poems.
Whitman brings this out in several other parts of his letter, suggesting that there is a sense of the "new" within the writing that people like himself and Emerson have undertaken. This "new" is something that reveals itself in all forms and is tied into the idea that the new nation is at a point where a new literary tradition can help to give voice to it. Whitman's argument is that the new type of people that are forming America, individuals who personify the boundless energy of the democratic spirit without stratification, are the ones who will compose this new sense of identity for the nation and it's up to the artists to bring this into reality:
Always waiting untold in the souls of the armies of common people, is stuff better than anything that can possibly appear in the leadership of the same. That gives final verdicts.
It is here, in what Whitman calls "the limitless foundations," where the hope for the literary and political future of America lies. In this, Whitman writes to Emerson, from one who wishes to continue the voice that another has already started to provide. In this, Whitman asserts there is one who claims his life devoted to "the making of poems."