He is using logic to show that America failed to keep her promise
It is effective because it clearly shows that the Constitution that was written for everybody was not duly followed.
Explanation:
Martin Luther King Jr made use of logos in his argument against the injustice and suppression faced by the blacks in America. He duly noted that the forefathers of America made a Constitution that all men had inalienable rights to life, property, liberty, etc but such was not extended to the black man.
