Amendments to the US Constitution are ratified by the states, and by the states alone. The federal government itself does not have a say at all. Amendments don't come along that often, and a lot of people don't realize this. What fewer people realize is that amendments can originate in the states as well; they do not have to be originally offered by the federal government. This method is in the Constitution but has not ever been implemented. If the states initiate an amendment, the federal government only tells the states what method will be used to ratify-- either by state legislatures, or by state conventions gathered for this purpose.
Emdrgreg