A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

U.S. Constitution
Parenthetical citations–reference list style: citing specific types of sources
Part II. Source Citation

The U.S. Constitution should be cited only in parenthetical citations; you need not include it in your reference list. Include the article or amendment, section, and, if relevant, clause. Use arabic numerals and, if you prefer, abbreviations for terms such as amendment and section.

P: (U.S. Constitution, art. 2, sec. 1, cl. 3)

(U.S. Constitution, amend. 14, sec. 2)

In many cases, you can include the identifying information in your text, but spell out the names of parts. Capitalize the names of specific amendments when used in place of numbers.

The U.S. Constitution, in article 1, section 9, forbids suspension of the writ “unless when in Cases of Rebellion or Invasion the public Safety may require it.”

The First Amendment protects the right of free speech.