U.S. constitutional amendment process

Background
Recognizing that future eras would face different sets of issues, challenges, and priorities, the Founding Fathers created an amendment process by which the Constitution could be altered. Article V of the Constitution grants this right, stating:

The Congress, whenever two-thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid, to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress.

Two ways to amend the Constitution
The founders offered two mechanisms for changing the Constitution. The first is for the proposed bill to pass both halves of the U.S. Congress (House and Senate) by a two-thirds majority in each. Once the bill has passed both houses, it then goes to the states. While the Constitution does not impose a time limit on states for which to consider the amendment, Congress frequently includes one (typically seven years). In order to become an amendment, the bill must receive the approval of three-fourths of the states (38 states). This approval can be generated through either a state convention or a vote of the state legislature. In either case, a majority vote is necessary for passage. Often, the proposed amendment specifies the route which is necessary.

The second method prescribed is for a Constitutional Convention to be called by two-thirds of the state legislatures (34 states), and for that Convention to propose one or more amendments. These amendments are then sent to the states to be approved by three-fourths of the legislatures or conventions. As of July 2006, this method has never been used. 

Role of the executive
Unlike the process by which a law is passed, the executive branch (the president) plays no official role in the amendment process. Presidents may call upon Congress to consider an amendment, but these requests carry no force of law.

Amendment process in practice
Because any amendment can be blocked by only thirteen states withholding approval (in either of their two houses), amendments are difficult to pass. In over two-hundred years since the ratification of the Constitution, over 10,000 amendments have been proposed, while only 27 have been added. The first ten of these (The Bill of Rights) secured passage almost immediately. The very difficulty of amending the Constitution greatly increases the importance of Supreme Court decisions interpreting the Constitution, because reversal of the Court's decision by amendment is difficult to obtain. 

Proposed constitutional amendments

 * Federal Marriage Amendment
 * Flag burning amendment

Articles

 * "The Amendment Process," The U.S. Constitution Online, March 21, 2006.
 * "Article V: Amending the Constitution," Exploring Constitutional Conflicts, 2001-2006.

Resources

 * United States Constitution