Wikipedia: "Checks and Balances"
In drafting the United States Constitution, the framers included features of many novel concepts including hard-learned historical lessons about checks and balances on power and the then-new concept of the separation of powers. Similar concepts were also prominent in the state governments of the United States. As colonies of Britain, the founding fathers felt that the American states had suffered an abuse of the broad power of the monarchy. As a remedy, the American Constitution limits the powers of the federal government through several means, in particular by dividing up the power of the government among three competing branches of government. Each branch checks the actions of the others and balances their powers in some way.
The independence of the executive and legislative branches is partly maintained by the fact that they are separately elected, and are held directly accountable to the voters.The legal mechanisms constraining the powers of the three branches depend a great deal on the popular sentiment of the people of the United States. Popular support establishes legitimacy, and makes possible the physical implementation of legal authority.