The source of this quotation is a journal kept by James McHenry (1753-1816) while he was a Maryland delegate to the Constitutional Convention. On the page where McHenry records the events of the last day of the convention, September 18, 1787, he wrote: “A lady asked Dr. Franklin Well Doctor what have we got a republic or a monarchy – A republic replied the Doctor if you can keep it.” Then McHenry added: “The Lady here alluded to was Mrs. Powel of Philada.” The journal is at the Manuscript Division of the Library of Congress.
And it turns out that we couldn’t
Waiting For Charlemagne 