There's a school of thought that considers the Declaration of Independence a more important document than the Constitution. These people claim that the Constitution can only properly be understood in light of the Declaration, and that the Declaration resolves any ambiguities that might be found in the Constitution. Abraham Lincoln supposedly thought that way, and today's acolytes of the Declaration point to his example. Lincoln's opposition to slavery, they note, was based on the Declaration's assertion that all men are created equal, a proposition not to be found in the Constitution. In this view, the Declaration is as much part of the country's fundamental law as the Constitution, if not more so.
It's easy to see why some people would prefer the Declaration. Unlike the Constitution, the Declaration appeals to "natural law," -- in Jefferson's words, "the Laws of Nature and Nature's God." The Constitution famously fails to mention God except in dating itself in the "Year of Our Lord" 1787. As already noted, Jefferson wrote that "all men are created equal." A lot of readers are more concerned about "created" than they are about "equal." They seem to think that we can't be equal unless we're created equal, and that equality depends on the conscious will of a creator who is simultaneously a lawgiver. I don't know if this is what Jefferson meant. He had issues with Christianity, disliked its supernatural aspects, but probably did believe in some sort of cosmic order. Just the same, I doubt whether we can translate "all men are created equal" into "all men are equal because God says so."
Moving right along, the Declaration asserts that men "are endowed by their Creator with certain unalienable Rights." Jefferson then lists "Life, Liberty and the pursuit of Happiness" as some of those rights ("among these" implying that the three he lists don't complete the set). I don't know if anyone has ever tried to find biblical sources for the unalienable Rights, but I'm not sure they'd be found in the Bible if anyone did try. Again, "Nature's God" is not necessarily, and indeed probably not the God of the Bible. "Nature's God" may in fact just be a figure of speech, and when paired with "the Laws of Nature" it looks like a redundancy. Again, I doubt that Jefferson meant that the Rights were unalienable by the will of God. He was more likely using the common Judeo-Christian idioms of his culture to say, "we have these rights because we are human beings."
The Declaration holds as self-evident that "to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed." Here Jefferson is moving out of God's shadow. He realized that any assertion of rights implies the existence of a guarantor. A right is worthless if it can't be enforced. If the unalienable Rights are based on the will of God, then God should be the guarantor. Instead, men have to institute governments and have to sign off themselves on the means of securing their rights. There's no waiting for God to smite those who dare alienate our rights; we have to do it ourselves. In more practical language: we assert these rights because we think they're ours as human beings, and we're going to back them up with force -- anyone who violates our rights is going to get beat down by us, not by God.
That's the point of the following: "whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it; and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness." The Declaration, of course, is only a justification of steps already taken. It's a propaganda document, and Jefferson has been building an intellectual basis for the American uprising against British rule. The rest of it is a delineation of abuses of power which justify the revolution. It's a long list because the Declaration committee realized that "Governments long established should not be changed for light and transient causes."
However, "when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce [mankind] under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security."
It should be clear enough that the Declaration is only a founding document to the extent that it declares the United States to be "Free and Independent States." On its face it is a guidebook for revolution. It tells us when we should rise up against oppression. As such it is extralegal, since there's nothing in the Constitution saying, "Please see the Declaration of Independence to clarify what we've written," or, "If anything in this Constitution goes against the Declaration of Independence, then we're wrong and we're sorry." You can use the Declaration to argue that something the government's up to is wrong, but that doesn't automatically win your case. Its assertion of natural law is not incorporated into the Constitution, though some might say that the Ninth Amendment implies natural rights when it declares that "the enumeration in the Constitution of certain rights, shall not be construed to deny or disparage others retained by the people." Justice Scalia presumes natural rights when he refers to "pre-existing rights" in last week's gun-rights case -- but would he concede a right to revolt if a government tried to take away our guns? Would he condone a revolt against the Bush Administration under any circumstance? If not, he should be more reticent about those "pre-existing rights."
British rule in America had become oppressive to a significant number of Americans without bringing rights into it. Parliament was holding back American economic development with its colonialist, mercantilist policies, which were meant to benefit people in the mother country at the colonies' expense. Taxation without representation was the tip of an iceberg of resentment, not all of it justified. Many colonists thought they were being treated like second-class citizens or worse. It was up to Parliament to accommodate the colonies on at least some points, and Britain got what it deserved for failing to do so. The revolutionaries needed to write a Declaration to prove that they had a right to rebel. The Declaration should be superfluous now, because we have a Constitution that defines our rights and constrains the government. If we want to live up to the ideals of the Declaration, then we should read the Constitution carefully and with our ancestors' readiness to rise up when government goes beyond its bounds. The Declaration of Independence is a great document, but it isn't the last word on the subject of rights and duties. As society evolves and globalizes, a new Declaration will probably be written as we learn what rights we want in a new environment, and a new Constitution will probably follow if we don't all die first.