King referenced that ""america has defaulted on this promissory note insofar as its citizens of color are concerned."" it was referring to the time when the words of constitution and declaration of independence written by the republic . In this note it was a promise that all men which includes black men and white men' they would be given a guarantee for their unalienable rights of life, liberty and pursuit of happiness. In this day, America has defaulted on this promissory note, insofar as her citizens of color are concerned.
By the above statement, King was metaphorically addressing the noncompliance of the government with the civil rights as stated in the US Constitution for all US citizens as stated in the (14th and 15th) Declaration of Independence and the Reconstruction Amendments which was without race discrimination as at when declared.
King was trying to argue that the US owed its black citizens because it has not kept the promises made to the general American people because it has failed to guarantee the rights of life, right of freedom, and right of the pursuit of happiness.
Instead, the black community had been given a blank check and are left unattended to.
