Walt Whitman has a lot of things to say about America and what it means to be American. In his poem "Song of Myself," one of the ways in which he defines what it means to be American is his exploration of the Democratic Self.
Whitman's Democratic Self is an idea that equates the individual with the universal. Consider, for instance, the following lines:
And these tend inward to me, and I tend outward to them,
And such as it is to be of these more or less I am,
And of these one and all I weave the song of myself. (327-9)
This quote comes at the end of a long list describing many people and scenes from diverse walks of life. Basically, what Whitman is saying here is that the self encompasses a diverse array of experiences and other selves. This idea is particularly important for Americans, who (theoretically) exist in a diverse society that relies upon a multitude of different opinions and beliefs to function. Indeed, this universal self draws upon the ideals of Democracy that America relies upon, and so the Democratic Self (the individual who exists within and draws upon multiple perspectives and personas) becomes a quintessentially American experience.
Whitman references this idea in later sections by referring to himself as "a kosmos" (497) and by claiming, "I am large, I contain multitudes" (1326). Thus, much of "Song of Myself" focuses on developing this American idea of the Democratic Self, the self that relies upon diversity of opinion, lifestyle, and experience for existence. Whitman's conception of the individual mirrors the ideals of American Democracy, which seeks to incorporate the lifestyles and cultures of many different people to exist. Of course, much could be done to criticize Whitman's idealistic view of both American society and of the individual, but the fact remains that the poet's democratic conception of what it means to be an American remains radical and intriguing to this day.