depends on where...
if you are doing the body of the constitution, then do it by article, and section
ie (U.S. Constitution, Art I, Sec 1) if you do a quote about the legislative branch.
or (U.S. Constitution, Preamble) if you do that opening
if you are quoting the amendments (U.S. Constitution, Amendment #)
This is very similar to how you would cite the bible as well. If any texts you use actually is separated into parts like that, such as a "Bill", then you can cite it like that using the text's own labels. a short hand would be (U.S. Constitution, I, 1) instead of Art. I and Sec. 1 respectively. this is also similar to citing peotry which is denoted by line numbers.
note, if your prof\teacher is a hardass, ask if this form of citation is okay, cuz some of them are hardasses.