From the Legal Information Institute: SEPARATION OF POWERS AND CHECKS AND BALANCES

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When the colonies separated from Great Britain following the Revolution, the framers of their constitutions were imbued with the profound tradition of separation of powers, and they freely and expressly embodied the principle in their charters.2 The theory of checks and balances, however, was not favored, because it was drawn from Great Britain, and, as a consequence, violations of the separation-of-powers doctrine by the legislatures of the states were commonplace prior to the convening of the Convention.3 Theory as much as experience guided the Framers in the summer of 1787.4

The doctrine of separation of powers, as implemented in drafting the Constitution, was based on several generally held principles: the separation of government into three branches, legislative, executive, and judicial; the conception that each branch performs unique and identifiable functions that are appropriate to each; and the limitation of the personnel of each branch to that branch, so that no one person or group should be able to serve in more than one branch simultaneously. To a great extent, the Constitution effectuated these principles, but critics objected to what they regarded as a curious intermixture of functions, in, for example, the veto power of the President over legislation and to the role of the Senate in the appointment of executive officers and judges and in the treaty-making process. It was to these objections that Madison turned in a powerful series of essays.5

Madison recurred to “the celebrated” Montesquieu, the “oracle who is always consulted,” to disprove the contentions of the critics. “[T]his essential precaution in favor of liberty,” that is, the separation of the three great functions of government, had been achieved, but the doctrine did not demand rigid separation. Montesquieu and other theorists “did not mean that these departments ought to have no partial agency in, or control over, the acts of each other,” but rather liberty was endangered “where the whole power of one department is exercised by the same hands which possess the whole power of another department.”6 That the doctrine did not demand absolute separation provided the basis for preservation of separation of powers in action. Neither sharply drawn demarcations of institutional boundaries nor appeals to the electorate were sufficient.7 Instead, the security against concentration of powers “consists in giving to those who administer each department the necessary constitutional means and personal motives to resist encroachments of the others.” Thus, “[a]mbition must be made to counteract ambition. The interest of the man must be connected with the constitutional rights of the place.”8

Institutional devices to achieve these principles pervade the Constitution. Bicameralism reduces legislative predominance, while the presidential veto gives to the President a means of defending his priorities and preventing congressional overreaching. The Senate’s role in appointments and treaties checks the President. The courts are assured independence through good-behavior tenure and security of compensation, and the judges through judicial review will check the other two branches. The impeachment power gives to Congress the authority to root out corruption and abuse of power in the other two branches. And so on.