[SMGFan]

thank you for the correction. Then the statement is based on ... (Is this correctly attributed?)
Well, Doctor, what have we got—a Republic or a Monarchy?”

  “A Republic, if you can keep it.” The response is attributed to BENJAMIN FRANKLIN—at the close of the Constitutional Convention of 1787, when queried as he left Independence Hall on the final day of deliberation—in the notes of Dr. James McHenry, one of Maryland’s delegates to the Convention.

[Olog-hai]

Seems to be correct; it appeared in the notes of Dr. James McHenry, one of the delegates from Maryland to the Constitutional Convention. It also appears in Barry Goldwater’s book, The Conscience of a Conservative.