MLK Jr's 'Promissory Note': Extended Metaphor

During college, my Jamaican roomate-friend Damian had memorized Martin Luther King, Jr.'s important "I Have a Dream" speech.  I was impressed again today (as I was in college) by the brilliant, extended metaphor MLK, Jr uttered almost 50 years ago, August 28, 1963:

This note was a promise that all men would be guaranteed the inalienable rights of life, liberty, and the pursuit of happiness.  It is obvious today that America has defaulted on this promissory note insofar as her citizens of color are concerned. Instead of honoring this sacred obligation, America has given the Negro people a bad check which has come back marked 'insufficient funds.' But we refuse to believe that the bank of justice is bankrupt. We refuse to believe that there are insufficient funds in the great vaults of opportunity of this nation.

Amazingly, we can watch his speech, all 17 minutes of it!