The concept of checks and balances is written into the U.S. Constitution. Checks and balances ensure that no branch of the government -- the judicial, executive or legislative branch -- obtains significantly more power than any other. The Constitution designed certain mechanisms, like veto powers and court reviews, that create a constant tug-of-war between the various branches of government. This tension ensures that they all "check" each other to create "balance."
Presidential Checks on Congress
The executive branch of the government -- occupied by the president -- has the ability to check the power of the legislative branch, which is the Congress. While Congress has the authority to originate and pass all laws, including those pertaining to funding, the president has the ability to veto such laws. Doing so means that the bill passed by Congress will not become law. In addition, the president has the authority to execute laws passed by Congress. In some circumstances, Congress can give instructions, but in others, the president has significant leeway in how he wants to execute the laws. As such, though Congress creates and passes laws, the president can also impart his influence over them.
Congressional Checks on the President
Though the president can check Congress, the checks and balances concept would be incomplete if Congress couldn't also check the president. Luckily, Congress has more than just the law-origination power in its arsenal. Even if a president vetoes a bill passed in Congress, the two houses can overturn the president's veto with a two-thirds majority vote in both houses. In addition, the Senate has the ability to vote on and reject federal officials appointed by the president. This power, in turn, gives the Congress some oversight in the execution of federal laws. Finally, if a president violates a serious law, Congress can impeach him or her and remove the chief executive from office.
Checks Within Congress
While Congress can check the other two branches of government, it also has the ability to administer checks within itself. Each of the houses of Congress -- the House of Representatives and the Senate -- represents different interests. When a law is originated, it must pass both houses to be sent to the president's desk. Each house, therefore, has its own veto power over the interests of the other house. In addition, the two houses have distinct types of authority. All spending bills, for example, must originate with the House. The Senate, meanwhile, has authority over treaties and presidential appointments in a way the House does not.
The judicial branch of government also has checks over the executive and legislative branches, and vice versa. The president, for example, has the authority to appoint all Supreme Court justices, though they all must be confirmed by the Senate. Congress can also amend the Constitution to avoid constitutional review decisions handed down by the Supreme Court. The court has its own authority, namely through the constitutional review process. In this situation, the court determines whether a law agrees with constitutional principles. If it doesn't, the court can strike down the law or a part of it.
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