According to the Declaration of Independence, the government gets its power to govern from the people that it governs. As the Declaration says,
Governments are instituted among Men, deriving their just powers from the consent of the governed.
This was an idea that derived from Enlightenment thinkers such as John Locke. These thinkers believed that people had natural human rights that they were born with. These were rights to such things as their life, their liberty, and their property. The problem was that, without government, other people could take these things away from them. People could rob, enslave, or kill others.
Because of this, people needed to have governments. What people did was to voluntarily give up some of their rights to a government in return for having their most important rights protected. A government was only legitimate, Enlightenment thinkers argued, when the people agreed to be ruled by it.
The writers of the Declaration of Independence borrowed their ideas from Locke and others like him. Therefore, they believed that government only justly held power if the people consented to give it that power.
The Declaration of Independence, of course, was intended by its principal author, Thomas Jefferson, to draw a marked contrast between the government it would establish and the one it would replace, with the latter characterized by the near-absolute power of monarchy. Jefferson and the other intellectual architects of American democracy (or republicanism) sought to distance the country they would forge from the distant autocratic one under whose yoke the North American colonies increasingly chafed. Key to establishing a government that would reflect the will of those over whom it governed was to ensure that this new political entity was truly representative of the population that would elect it. In drafting the subsequent Constitution of the United States of America, James Madison would similarly enshrine in this seminal document the principles of representative democracy. It was, though, in the Declaration of Independence where the crucial concept of consent of the governed was best exemplified. It is in the second paragraph that Jefferson spells-out precisely the source of the government's power: the people. In that paragraph, Jefferson wrote:
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.--That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, --That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government . . ." [Emphasis added]
This section is the heart of American democracy. The system of representative government being established would be firmly rooted in the will of the people. No chief executive, no senator or congressman, and no justice would sit above the population, exercising unquestioned power. The president and the members of the legislature would serve at the pleasure of the populace -- not the other way around.
Montesquieu was another political philosopher who influenced the shaping of the United States Constitution. Although he died in 1755, his ideas about the separation of powers were adopted and applied by James Madison, the principal author of the United States Constitution.
The reason behind the separation of powers concept is essentially pessimistic: Left unchecked, one branch of a government will seek to eventually dominate the others. Madison and the other constitutional framers sought to balance the three branches of government (legislative, executive, judicial) in order to “keep them honest”). For example, the president can veto Congress, but Congress can override the president, an the courts can declare a law unconstitutional.
Much of this distrust of governmental power grew out of centuries of monarchial rule in the European countries that populated America in the seventeenth and eighteenth centuries.
The downside of this system can be seen today with what we refer to as “gridlock” in Washington. Without a dominant party, neither conservatives nor liberals can muster enough power to get much done legislatively. While the system keeps one branch from bullying the others, it also makes it very tough to get anything accomplished when you have different parties in power in the White House and on Capitol Hill.