If you are citing a web page with information about the Declaration or Constitution (even if it also has the document on it), you should use the Web page citation type.
If you are citing the Declaration or Constitution itself, do not cite it in the works cited list. Both the Declaration and the Constitution are considered well-known documents that are only cited in a parenthetical reference.
In your text, do not underline or use quotation marks for the words Declaration of Independence or Constitution of the United States, just use a parenthetical reference, as below
...in the Declaration of Independence (US 1776).
...In the Constitution of the United States, Article II refers to the "...." (sec.1, cl.3)
...in the U.S. Constitution (art. 2, sec. 1, cl.3.).
...slavery was finally abolished in December 1865 (US Const., amend. XIII).
