A Constitutional republic is a republic that operates under a system of separation of powers, where both the chief executive and members of the legislature are elected by the citizens and must govern within an existing written constitution.
In a constitutional republic, executive and legislative powers are separated by vote into distinct branches; and the judiciary is independent. A state is constitutional if a constitution limits and separates the government's powers. The United States of America and the French Republic are examples of constitutional republics.
The difference of the Constitutional Republic with other forms of government is that the executive power must be legitimated by vote, without being able to be appointed by the legislature and has an independent judicial administration, no power can exercise the executive, legislative and judicial powers at the same time, only the Constitutional Republic legitimizes executive power.
The United States of America is the oldest Constitutional Republic in the world and represents the first integral experiment in this Form of State conceived. With the ratification of the Constitution of the United States, the Federal Republic established for the first time in history the representative democracy in the form of government of the state, due to separate and periodic elections for both the chief executive and the members of the legislature.
According to James Woodburn in The American Republic and Its Government, "the constitutional republic with its limitations on popular government is clearly involved in the United States Constitution, as seen in the election of the President, the election of the Senate and the appointment of the Supreme Court." Woodburn says that in a constitutional republic (representative democracy), unlike direct democracy, people are regulated not only by electing officials but also by making laws through their representatives. There is a Bill of Rights in the US Constitution that protects certain individual rights. The individual rights enumerated in the Bill of Rights cannot be voted on by the majority of citizens if they wish to oppress a minority that does not agree with the restrictions on freedom they wish to impose. To eliminate these rights it would be required that government officials overcome the rigid constitutional controls. The Constitution of the United States is the most difficult to amend in the world; to amend it requires a two-thirds majority proposal of both chambers of the Federal Congress. After the official proposal, a constitutional amendment must be ratified by the legislatures of, or by conventions within, at least three quarters of all states.
Until the eighteenth century it was argued that democracy was only possible in small Italian and Greek cities, where citizens in assemblies voted and made the law and the policies of the government and at the same time the citizens directly and separately elected the highest magistrates of the different political bodies of the Athenian democracy through lot and voting processes; the US Federal Constitutional Republic added to the direct and separate election of the highest magistrates of the State, the liberal representation inspired by the British parliamentary system, resulting in the possibility of democracy being possible in a large territory due to the delegation in electoral districts of the people's representatives:
A representative democracy, where the right of election is well secured and regulated & the exercise of the legislative, executive and judiciary authorities, is vested in select persons, chosen really and not nominally by the people, will in my opinion be most likely to be happy, regular and durable.
A great democratic republic has never been seen until now. It would be an insult to republics to give this name to the oligarchy that reigned over France in 1793. The United States alone presents this new spectacle.
John Adams defines a constitutional Republic as "a government of the law, and not the government of the people." In the same way, the power of government officials is controlled so that no one keeps all executive, legislative and judicial powers. On the contrary, these powers are divided into discrete sectors that serve as a system of checks and balances. A constitutional republic has the form of "no person or group can obtain absolute power". 
- Woodburn, James Albert. The American Republic and Its Government: An Analysis of the Government of the United States, G. P. Putnam, 1903. pp. 58–59.
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When the idea of the rule of law is interpreted as a principle of constitutionalism, ...
- Peacock, Anthony Arthur (2010-01-01). Freedom and the Rule of Law. Rowman & Littlefield. ISBN 9780739136188.
The rule of law is fundamental to all liberal constitutional regimes...
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