how much of the senate has to agree to ratify a treaty
Treaty power is a co-ordinated effort between the Executive branch and the Senate. The President may form and negotiate, 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 it is ratified, it becomes binding on all the states under the Supremacy Clause. While the House of Representatives does not vote on it at all, the requirement for the Senate's advice and consent to ratification makes it considerably more difficult to rally enough political support for international treaties.