Popular sovereignty exists in the Tenth Amendment of the Constitution. This is the amendment that gives the states the powers not reserved for the federal government. This places issues that directly affect the people at the state level in the hands of the state governments; these governments are theoretically more agile and more responsive to the people living within the state.
The Constitution has several amendments dealing with political equality. The Fourteenth Amendment protects citizenship rights for all and was mainly ratified in order to grant African Americans legal protections after the Civil War. The Fifteenth Amendment gave all African American males over the age of twenty-one the right to vote, though this amendment was often flouted in the Jim Crow South. The Nineteenth Amendment granted women the right to vote. The Twenty-Fourth Amendment did away with poll taxes, thus making voting free and available for all. The Twenty-Sixth Amendment gave the right to vote to anyone eighteen years of age or older. This was placed in the Constitution after many young people protested that they were old enough to go to Vietnam but not old enough to pick the political leaders who made that choice.
The Constitution also has several amendments that handle liberty and democracy. The Bill of Rights has safeguards that preserve what are considered the "inalienable rights" of all people. The Thirteenth Amendment abolished slavery in the United States. The Seventeenth Amendment created the direct election of senators; before this, senators were chosen by state legislatures.