Does the Constitution have legal weight?
The US Constitution and the subsequent amendments are law of the land, as well as the US Code, the rulings by the Supreme Court, laws passed by Congress and signed by the President, laws passed by Congress which over-rule the President's veto, Executive Orders, the Code of Federal Regulations, US Statutes, rulings by the US Court of Appeals, rulings by the US District Courts, and then each State's constitutions, the rulings by each state's supreme court, each state's statutes, each state's regulations, and then municipal law for each city or town in those states, all have legal weight.
The Declaration of Independence had only one purpose: to declare the independence of 13 colonies from Great Britain. Once finished, each former colony worked on their constitutions, and then worked on the Articles of the Confederation, and later the US Constitution when they realized that the Articles weren't working so good.