Since the U.S. Constitution sets down the principles upon which the government is based, you might want to cite the document in a research paper discussing laws, history or even ethical concepts. According to the sixth edition of the "Publication Manual of the American Psychological Association," APA style papers follow much of the format established in the Bluebook of legal documentation, including citations for the Constitution.
In-text citations indicate the words or ideas stemming from the Constitution either in the written phrase or in a parenthetical citation after the borrowed information. The Constitution is abbreviated "U.S. Const." (without the quotation marks). Then the part number appears, if any, after "pt." (without the quotation marks). The reader needs to know which amendment (amend.) or article (art.) followed by the number in Roman numerals. If you also reference a particular section, type a comma and then the section symbol that looks like two intertwined capital S's and then the numeral. The parenthetical citation for an amendment follows this example: (U.S. Const. amend. IX). A reference to a section within an article looks like this: (U.S. Const. art. II, [section symbol] 2). To document a constitution other than the U.S. document, such as a state constitution, abbreviate the name to begin the citation and include the relevant portions, like this example: (Minn. Const. pt. 3, art. XII.)
On the References page, the information is the same, but the parentheses are removed.
A date is not needed for portions of a constitution currently in effect. If you cite a concept that has been repealed or otherwise changed, however, indicate that information and the year the change occurred in the parenthetical citation. For example, (Minn. Const. pt. 3, art. XII, repealed 1988).
On the Reference page, the repeal date is placed in parentheses before the period at the end, as in this example: Minn. Const. pt. 3, art. XII (repealed 1988).