Technically, the Senate does not ratify treaties, but approves a resolution of ratification.
A 2/3 vote is needed.
senate.gov
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United States
In the US, the treaty power is a coordinated effort between the Executive branch and the Senate. The President may form and negotiate a treaty, but the treaty must be advised and consented to by a two-thirds vote in the Senate. Only after the Senate approves the treaty can the President ratify it. Once a treaty is ratified, it becomes binding on all the states under the Supremacy Clause. While the United States House of Representatives does not vote on it at all, the requirement for Senate advice and consent to ratification makes it considerably more difficult in the US than in other democratic republics to rally enough political support for international treaties. Also, if implementation of the treaty requires the expenditure of funds, the House of Representatives may be able to block, or at least impede, such implementation by refusing to vote for the appropriation of the necessary funds.
In the US, the President usually submits a treaty to the Senate Foreign Relations Committee (SFRC) along with an accompanying resolution of ratification or accession. If the treaty and resolution receive favorable committee consideration (a committee vote in favor of ratification or accession) the treaty is then forwarded to the floor of the full U.S. Senate for such a vote. The treaty or legislation does not apply until it has been ratified. A multilateral agreement may provide that it will take effect upon its ratification by less than all of the signatories.[1] Even though such a treaty takes effect, it does not apply to signatories that have not ratified it. Accession has the same legal effect as ratification. Accession is a synonym for ratification for treaties already negotiated and signed by other states.[2] An example of a treaty to which the U.S. Senate did not advise and consent to ratification is the Treaty of Versailles, which was part of the resolution of the First World War. |
