There are essentially two ways spelled out in the Constitution for how to propose an amendment. One has never been used.
The first method is for a bill to pass both houses of the legislature, by a two-thirds majority in each. Once the bill has passed both houses, it goes on to the states. This is the route taken by all current amendments. Because of some long outstanding amendments, such as the 27th, Congress will normally put a time limit (typically seven years) for the bill to be approved as an amendment (for example, see the 21st and 22nd).
The second method prescribed is for a Constitutional Convention to be called by two-thirds of the legislatures of the 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. This route has never been taken, and there is discussion in political science circles about just how such a convention would be convened, and what kind of changes it would bring about.
Regardless of which of the two proposal routes is taken, the amendment must be ratified, or approved, by three-fourths of states. There are two ways to do this, too. The text of the amendment may specify whether the bill must be passed by the state legislatures or by a state convention. See the Ratification Convention Page for a discussion of the make up of a convention. Amendments are sent to the legislatures of the states by default. Only one amendment, the 21st, specified a convention. In any case, passage by the legislature or convention is by simple majority.
The Constitution, then, spells out four paths for an amendment:
- Proposal by convention of states, ratification by state conventions (never used)
- Proposal by convention of states, ratification by state legislatures (never used)
- Proposal by Congress, ratification by state conventions (used once)
- Proposal by Congress, ratification by state legislatures (used all other times)
It is interesting to note that at no point does the President have a role in the formal amendment process (though he would be free to make his opinion known). He cannot veto an amendment proposal, nor a ratification. This point is clear in Article 5, and was reaffirmed by the Supreme Court in Hollingsworth v Virginia (3 US 378 ):
The negative of the President applies only to the ordinary cases of legislation: He has nothing to do with the proposition, or adoption, of amendments to the Constitution.