In the "I Have a Dream" speech by Martin Luther King, Jr., who was the intended audience and how do they affect the writing?

On August 28, 1963, Martin Luther King, Jr., was one of the leaders of the March on Washington for Jobs and Freedom, which drew 200,000 participants of all races. At that time, the gathering was the largest crowd ever assembled in Washington, D.C. Politicians, pastors, entertainers, civil rights leaders, and people from around the country attended the organized day-long rally and made up the audience for King's speech. The crowd included both blacks and whites, mostly those who were sympathetic to the civil rights movement. However, King certainly realized that his speech would be circulated widely after the event through television and print media, so his broader audience was the country as a whole. 

The fact that his audience included not only blacks, who were the targets of racial discrimination, but also whites, affected the content of King's speech. In the beginning of his speech, he understandably draws in the black members of his audience first. He points out how "America has given the Negro a bad check, a check which has come back marked 'insufficient funds,'" showing how the demands of the blacks were just. He specifically addresses people of his own race when he says

"there is something that I must say to my people who stand on the warm threshold which leads into the palace of justice."

He calls on blacks to reject violence and to resist the temptation to distrust whites. He encourages them not to be satisfied and not to give up until their dreams are realized.

Next, he transitions his remarks to include whites as well as blacks. He makes this transition by asserting that his dream is rooted in the American Dream, something that all races aspire to. When he refers to his four little children, he transcends race, since the dreams parents have for their children are the same for people of any color. He speaks of "all God's children" and looks forward to a time when

"little black boys and black girls will be able to join hands with little white boys and white girls and walk together as sisters and brothers."

In the final paragraph, he envisions the day when "black men and white men, Jews and Gentiles, Protestants and Catholics, will be able to join hands and sing." 

Having an audience of both blacks and whites affected King's speech by making it inclusive and broad in scope rather than a speech that would narrowly address blacks. Because people from all races were listening and would be listening, King strove for a unifying tone in his address.