Martin Luther King, Jr.'s "I Have a Dream" speech is the form of discourse known as Argumentation.
In an argument-driven piece, speakers present evidence in order to convince their audience of the logic of their points of persuasion on a certain topic. In other words, the speaker urges his listeners to agree with his points so their minds change and positive action occurs.
Certainly, change and action are what Dr. Martin Luther King sought when he delivered his speech in Washington, D. C. Certainly, too, his style of delivery was persuasive in his mention of injustices and in his tone of voice and use of rhetorical devices.
One of Dr. King's arguments is his reference to the United States Constitution in which it is stated that all men are guaranteed the "unalienable Rights" of "Life, Liberty, and the pursuit of Happiness." Using the metaphor of "a promissory note" for this promise of the Constitution, King argues that the "Negro people have received a bad check," meaning they have not been offered the same opportunities as other American citizens. He cites segregation as the main prohibition of opportunities for African Americans. He argues the promises and ideals of the Constitution can only be mandated through an equivalent promise and commitment to the cause of Civil Rights.