Before King launches into his speech, he first establishes his credibility with the audience—using ethos to support his rhetoric. First, at the very beginning of the speech, he calls the March on Washington for Jobs and Freedom "what will go down in history as the greatest demonstration for freedom in the history of our nation." King affirms the gravity and importance of the moment, which lends credence to his argument. Then, he affirms his prominence in the history of racial justice by comparing himself to Abraham Lincoln:
Fivescore years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation. This momentous decree came as a great beacon light of hope to millions of Negro slaves who had been seared in the flames of withering injustice. It came as joyous daybreak to end the long night of their captivity. But one hundred years later, the Negro is still not free [...]
King, giving his speech in front of the Lincoln Memorial, connects the struggle of the racial justice moment in 1963 directly to the abolition of slavery in 1863. He claims that he is continuing the same fight that Lincoln fought, a hundred years before: this lends credibility to his argument. King also uses language reminiscent of Lincoln. He says that Lincoln signed the Emancipation Proclamation "fivescore years ago," recalling the famous first line of the Gettysburg Address: "Four score and seven years ago..." By comparing himself to Lincoln and connecting their respective struggles, King asserts himself as an important and powerful figure in the history of racial justice, just as Lincoln was.
It was especially important for King to use ethos in his speech. King spoke to 250,000 people in front of the Lincoln Memorial who attended the March on Washington, the vast majority of whom would have already agreed with and trusted him—but the speech was also televised broadly and seen by millions of Americans, many of whom disagreed with his arguments for racial justice. Speaking on such a large stage, it was essential for King to establish his credentials before starting his speech, in order to invite the general public to engage with what he had to say.