The section of the United States Constitution is represented in the Works Cited list as follows:
United States Constitution. Art./Amend. XII, Sec. 3.
The intext citation would follow this pattern:
(US Const. amend. XII, sec. 3)
If a constitution is published in a named edition, treat it like the title of a book:
The Constitution of the United States: A Transcription . National Archives , United States National Archives and Records Administration, 28 Feb. 2017, www.archives.gov/founding-docs/constitution-transcript#toc-article-iv- .
The Constitution of the United States, with Case Summaries . Edited by Edward Conrad Smith, 9th ed., Barnes and Noble Books, 1972.
References to the United States Constitution in your prose should follow the usual styling of titles of laws:
But your in-text reference should key readers to the appropriate entry:
( Constitution of the United States, with Case Summaries )
For more information, also take a look at Purdue Owl’s Writing Lab page on MLA FAQ’s which also includes a passage on the how to cite the US constitution.