The Bill of Rights provides an important broad guarantee to the states regarding the limits of the powers of the national government and the essentially unlimited reserve of powers that the states may claim. Amendment 10 the last of the original ten amendments that constitute the Bill of Rights states:
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people."
This "reserved powers clause" is fundamental to the ability of the states to formulate and adopt their own constitutions and laws within the rubric of the U.S. Constitution.
Because the U.S. Constitution remains the fundamental constraint on the power of the states within the federal system, new constraints on state powers can and have come in the form of additional amendments to the Constitution. The most fundamental changes were set in motion by the Civil War. Amendments 13, 14, and 15, ratified in the years following the end of hostilities, placed new or reemphasized existing constraints on the states, including the prohibition on slavery, the guarantee of due process of the law for all individuals, and the legal guarantee of voting rights for freed slaves and their descendents. It took the better part of the following century to enforce the 14th and 15th Amendments, an illustration of the ability of the states to use the reserved powers to resist efforts to bring them into compliance with national mandates.
Later amendments prohibited unjust or undemocratic practices in the various states, or expanded the voting franchise to new groups. The 19th Amendment guaranteed women the right to vote throughout the country. The 24th amendment outlawed the poll tax, which tended to disenfranchise blacks and other minorities, as well as poor whites. The 26th lowered the legal voting age to 18 years.