WASHINGTON — New Hampshire Sen. Kelly Ayotte on Monday joined fellow Republican Sen. Rob Portman of Ohio to oppose Senate ratification of the controversial Law of the Sea Treaty.

The two senators, who happen to be mentioned as possible running mates for Republican presidential candidate Mitt Romney, became the final two votes needed to ensure the treaty will not be ratified.

The Constitution requires 67 affirmative votes for the Senate to ratify a treaty. Thirty-one senators had previously signed a letter in opposition and a 32nd senator announced his opposition to the pact.

“Portman and Ayotte's announcement makes efforts to ratify the Law of the Sea Treaty dead in the water in this Congress,” according to a joint statement from their offices.

The treaty would establish a system for resolving disputes in international waters and recognize sovereign rights over a country's continental shelf out to 200 nautical miles and beyond, if a country can provide evidence to substantiate its claims, according to the Huffington Post.

The United States has abided by the rules of the treaty since the Reagan administration but is the only major nation that has refused to sign the treaty.

In a letter, the two senators cited concerns about the breadth and ambiguity of the treaty and risks to U.S. sovereignty.

Portman and Ayotte wrote to Senate Majority Leader Harry Reid, “We simply are not persuaded that decisions by the International Seabed Authority and international tribunals empowered by this treaty will be more favorable to U.S. interests than bilateral negotiations, voluntary arbitration, and other traditional means of resolving maritime issues.

“No international organization owns the seas, and we are confident that our country will continue to protect its navigational freedom, valid territorial claims and other maritime rights,” they wrote.

U.S. Sen. Jim DeMint, R-S.C., and other conservatives have reportedly led a campaign against the treaty.