Two Republican senators declared their opposition on Monday to the international agreement, bringing the total number of Senate opponents to 34 – enough to sink the measure. A two-thirds majority of 67 votes was required for ratification.

Sens. Kelly Ayotte (R-N.H.) and Rob Portman (R-Ohio) pushed the opposition movement over the top, citing concerns about U.S. sovereignty.

In a letter to Senate Majority Leader Harry Reid (R-Nev.), the two Armed Services Committee members declared: “No international organization owns the seas.”

“We are confident that our nation will continue to protect its navigational freedom, valid territorial claims and other maritime rights,” they said.

The treaty, established in the early 1980s to govern the use of international waterways and undersea resources, has eluded Congress for decades, despite repeated attempts to ratify it amid strong support among Pentagon commanders.

The latest attempt was spearheaded by Senate Foreign Relations Committee Chairman John Kerry (D-Mass.), who scheduled a series of hearings on the issue over the past three months. And on Monday, a Kerry spokeswoman signaled the senator still plans to push for ratification – but not until after the Nov. 3 elections.

“It’s not news to anyone that right now we’re in the middle of a white-hot political campaign season, where ideology is running in overdrive,” Kerry spokeswoman Jodi Seth said in a statement. “No letter or whip count changes the fact that rock-ribbed Republican businesses and the military and every living Republican secretary of state say that this needs to happen.”

In May, Defense Secretary Leon Panetta and Secretary of State Hillary Clinton appeared before the committee to urge ratification. “We are the only permanent member of the U.N. Security Council that is not a party to it,” Panetta said in his prepared testimony. “This puts us at a distinct disadvantage when it comes to disputes over maritime rights.”

Over the past several weeks, Ayotte and Portman – both believed to be on presumptive Republican presidential nominee Mitt Romney’s short list of potential running mates – were targeted by conservative advocacy groups, including Heritage Action for America, in a lobbying push to scuttle the treaty. And on Monday, Heritage put out a celebratory statement heralding the treaty’s apparent demise.

“America had little to gain through accession to the Law of the Sea Treaty – but much to lose,” said the group’s CEO, Michael Needham.