Is the U.S. Constitution poetry? Maybe, maybe not. However, we do know that the Preamble is best memorized through song. Moreover, even this English major didn’t quite grasp structure until her Con Law class.
We enjoyed, although did not entirely buy, Q&A: Burt Neuborne on “Madison’s Music,” a Bold New Approach to Interpreting the Constitution, National Book Review (May 2016).
See also the ambitious The U.S. Constitution Is a Poem, (that certainly settles the question), Geoffrey Wessel, Overthinking.com (September 17, 2015).
Is it fiction? Kind of, yes. See Constitutional Law as Fiction: Narrative in the Rhetoric of Authority, Lash Larue, Penn State University Press (1995).