When a person gains power over other persons — the political power to force other persons to do his bidding when they do not believe it right to do so — it seems inevitable that a moral weakness develops in the person who exercises that power. It may take time for this weakness. It was Lord Acton, who said: "All power tends to corrupt; absolute power corrupts absolutely."
Liberals promote constitutionalism in many different forms. The social contract, separation of powers and the rule of law.
Jacques Rousseau , in1762 came up wth the Social Contract, Or Principles of Political Right, outlined a different version of contract theory, based on the conception of popular sovereignty, defined as indivisible and inalienable — this last trait explaining Rousseau's aversion for representative democracy and his advocacy of direct democracy. Rousseau's theory has many similarities with the individualist Lockean liberal tradition, but also departs from it on many significant points. For example, his theory of popular sovereignty includes a conception of a "general will", which is more than the simple sum of individual wills: it is thus collectivist or holistic, rather than individualist. As an individual, Rousseau argues, the subject can be egoist and decide that his personal interest should override the collective interest. However, as part of a collective body, the individual subject puts aside his egoism to create a "general will", which is popular sovereignty itself. Popular sovereignty thus decides only what is good for society as a whole:
The social contract can be reduced to the following terms. Each of us puts his person and all his power in common under the supreme direction of the general will; and in a body we receive each member as an indivisible part of the whole
Hence, Rousseau's infamous phrase that man must "be forced to be free" should be understood as such: since individual subjects resign their free will. Rousseau's version of the social contract is the one most often associated with the term "social contract" itself. His theories had an influence on both the 1789 French Revolution.
Hobbes's theory, to form popular sovereignty; since the indivisible and inalienable popular sovereignty decides what is good for the whole, then if an individual lapses back into his ordinary egoism, he shall be forced to listen to what they decided as a member of the collectivity.
Another way of limiting power of a government is to separate the powers. Montesquieu described division of political power among an executive, a legislature, and a judiciary. He based this model on the British constitutional system, in which he perceived a separation of powers among the monarch, Parliament, and the courts of law. Subsequent writers have noted that this was misleading, since Great Britain had a very closely connected legislature and executive, with further links to the judiciary. But in Montesquieu's time, the political connection between Britain's Parliament and the monarch's Ministry was not as close as it would later become.
The Delegates were also very sharply divided.The Delegates also agreed with Montesquieu. Montesquieu did specify that "the independence of the judiciary has to be real, and not apparent merely". "The judiciary was generally seen as the most important of powers, independent and unchecked", and also considered the least dangerous. Some politicians decry judicial action against them as a "criminalization" of their behavior, but such "criminalization" may be seen as a response to corruption, collusion, or abuse of power by these politicians.
The rule of law can limit the power of a government because they must follow all laws they set down and they are not above the law. Thomas Paine stated in his pamphlet Common Sense (1776): "For as in absolute governments the king is law, so in free countries the law ought to be king; and there ought to be no other."
In conclusion liberals fear power because it can take from personal liberties, and power can corrupt anybody even if they believe that it won’t and they suggest constitutionalism because is a good way to limit the power of the government and to protect each person’s personal liberties.