Bluebook Rule (21st): 11
Law Review Typeface: SmallCaps
Rule 11 lays out the citation method for constitutional sections. You should cite to the United State federal constitution by "U.S. Const." and cite to state constitutions by "[abbreviated state name] Const."
Examples:
U.S. Const. art. 1, § 9, cl. 9.
Tex. Const. art. 1, § 3.