The Enlightenment philosopher Montesquieu coined the phrase “trias politica,” or separation of powers, in his influential 18th-century work “Spirit of the Laws.” His concept of a government divided into legislative, executive and judicial branches acting independently of each other inspired the framers of the U.S. Constitution, who vehemently opposed concentrating too much power in any one body of government.

In the Federalist Papers, James Madison wrote of the necessity of the separation of powers to the new nation’s democratic government: “The accumulation of all powers, legislative, executive and judiciary, in the same hands, whether of one, a few, or many, and whether hereditary, self-appointed, or elected, may justly be pronounced the very definition of tyranny.”

According to Article I of the Constitution, the legislative branch (the U.S. Congress) has the primary power to make the country’s laws. This legislative power is divided further into the two chambers, or houses, of Congress: the House of Representatives and the Senate.

Members of Congress are elected by the people of the United States. While each state gets the same number of senators (two) to represent it, the number of representatives for each state is based on the state’s population.

Therefore, while there are 100 senators, there are 435 elected members of the House, plus an additional six non-voting delegates who represent the District of Columbia as well as Puerto Rico and other U.S. territories.

In order to pass an act of legislation, both houses must pass the same version of a bill by majority vote. Once that happens, the bill goes to the president, who can either sign it into law or reject it using the veto power assigned in the Constitution.

In the case of a regular veto, Congress can override the veto by a two-thirds vote of both houses. Both the veto power and Congress’ ability to override a veto are examples of the system of checks and balances intended by the Constitution to prevent any one branch from gaining too much power.

Article II of the Constitution states that the executive branch, with the president as its head, has the power to enforce or carry out the laws of the nation.

In addition to the president, who is the commander in chief of the armed forces and head of state, the executive branch includes the vice president and the Cabinet; the State Department, Defense Department and 13 other executive departments; and various other federal agencies, commissions and committees.

Unlike members of Congress, the president and vice president are not elected directly by the people every four years, but through the electoral college system. People vote to select a slate of electors, and each elector pledges to cast his or her vote for the candidate who gets the most votes from the people they represent.

In addition to signing (or vetoing) legislation, the president can influence the country’s laws through various executive actions, including executive orders, presidential memoranda and proclamations. The executive branch is also responsible for carrying out the nation’s foreign policy and conducting diplomacy with other countries, though the Senate must ratify any treaties with foreign nations.

Article III decreed that the nation’s judicial power, to apply and interpret the laws, should be vested in “one supreme Court, and in such inferior Courts as the Congress may from time to time ordain and establish.”

The Constitution didn’t specify the powers of the Supreme Court or explain how the judicial branch should be organized, and for a time the judiciary took a back seat to the other branches of government.

But that all changed with Marbury v. Madison, an 1803 milestone case that established the Supreme Court’s power of judicial review, by which it determines the constitutionality of executive and legislative acts. Judicial review is another key example of the checks and balances system in action.

Members of the federal judiciary—which includes the Supreme Court, 13 U.S. Courts of Appeals and 94 federal judicial district courts—are nominated by the president and confirmed by the Senate. Federal judges hold their seats until they resign, die or are removed from office through impeachment by Congress.

In addition to the specific powers of each branch that are enumerated in the Constitution, each branch has claimed certain implied powers, many of which can overlap at times. For example, presidents have claimed exclusive right to make foreign policy, without consultation with Congress.

In turn, Congress has enacted legislation that specifically defines how the law should be administered by the executive branch, while federal courts have interpreted laws in ways that Congress did not intend, drawing accusations of “legislating from the bench.”

The powers granted to Congress by the Constitution expanded greatly after the Supreme Court ruled in the 1819 case McCulloch v. Maryland that the Constitution fails to spell out every power granted to Congress.

Since then, the legislative branch has often assumed additional implied powers under the “necessary and proper clause” or “elastic clause” included in Article I, Section 8 of the Constitution.

“In framing a government which is to be administered by men over men, the great difficulty is this: You must first enable the government to control the governed; and in the next place, oblige it to control itself,” James Madison wrote in the Federalist Papers. To ensure that all three branches of government remain in balance, each branch has powers that can be checked by the other two branches. Here are ways that the executive, judiciary, and legislative branches keep one another in line:

· The president (head of the executive branch) serves as commander in chief of the military forces, but Congress (legislative branch) appropriates funds for the military and votes to declare war. In addition, the Senate must ratify any peace treaties.

· Congress has the power of the purse, as it controls the money used to fund any executive actions.

· The president nominates federal officials, but the Senate confirms those nominations.

· Within the legislative branch, each house of Congress serves as a check on possible abuses of power by the other. Both the House of Representatives and the Senate have to pass a bill in the same form for it to become law.

· Once Congress has passed a bill, the president has the power to veto that bill. In turn, Congress can override a regular presidential veto by a two-thirds vote of both houses.

· The Supreme Court and other federal courts (judicial branch) can declare laws or presidential actions unconstitutional, in a process known as judicial review.

· In turn, the president checks the judiciary through the power of appointment, which can be used to change the direction of the federal courts

· By passing amendments to the Constitution, Congress can effectively check the decisions of the Supreme Court.

· Congress can impeach both members of the executive and judicial branches.

Separation of Powers, The Oxford Guide to the United States Government.
Branches of Government, USA.gov.
Separation of Powers: An Overview, National Conference of State Legislatures.