The 7 th edition of the MLA handbook has this to say about citing the U.S. Constitution:
"In general, do not italicize or enclose in quotation marks the title of laws, acts, and similar documents in either the text or the list of works cited (Declaration of Independence, Constitution of the United States, Taft-Hartley Act). Such titles are usually abbreviated, and the works are cited by sections. The years are added if relevant" (205).
Because these directives aren’t very specific, you can use the following example as a guide for the Works Cited entry:
U.S. Constitution. Art./Amend. XII, Sec. 3.
You need only provide either the article number or the amendment number as appropriate.
The complementary parenthetical citation is written as (US Const. amend. XII, sec. 3). You might also reference the U.S. Constitution in the sentence itself and only provide the amendment and section number in the parentheses at the end of the sentence.