The Constitution is a contract betwee government and governed; if one side, the government, in the situation you describe, abrogates or breeches that contract, their legal source of authority under that constitution is equally null and void, and they no longer have any more legitimate authority than any other violent criminal gang [though they'll loudly proclaim otherwise.] As such, with no legitimate government in place, you'll have to live by and enforce your own moral code. Pity about the U.S. Constitution- the American experiment in participatory democracy was a noble experiment while it lasted.
I wonder what date historians will finally pick to note as for its epitaph. It's likely that any between 22 November 1963 and 11 September 2001 could serve, evin if the corpse hadn't completely putrified yet.
-archy-/-