The chances of the United States ratifying a new treaty to regulate the multibillion-dollar global arms trade slipped as senators announced enough opposition to block the move.
Worries about domestic gun control are behind senators' opposition to the pact, which North Korea, Syria and Iran also oppose but for other reasons.
One day after the UN General Assembly overwhelmingly approved the treaty, Republican Sen. Mike Lee said it was "deeply flawed" and became the 35th senator to endorse a resolution of opposition. The US Constitution requires two-thirds of the Senate - 67 votes - to ratify a treaty.
In a statement that reflected the strong objections of gun rights advocates, Lee said, "I have great concerns that this treaty can be used to violate the Second Amendment rights of American citizens, and do not believe we should sign any treaty that infringes on the sovereignty of our country."
The Second Amendment to the Constitution guarantees the right to bear firearms. But it is longstanding legal principle that no treaty can override the Constitution or US laws.
After some hesitation last year, the United States joined 153 nations in backing the treaty that proponents say will keep weapons out of the hands of terrorists and human rights abusers. Iran, North Korea and Syria, which face international arms embargoes, voted against the agreement.
Enforcement is at the discretion of the countries that ratify the treaty.
Opponents of the treaty say any restriction on international arms or gun sales will lead to greater gun control in the United States.
The treaty prohibits countries that ratify it from exporting conventional weapons if they violate arms embargoes; if they promote acts of genocide, crimes against humanity or war crimes; or if they could be used in attacks against civilians or schools and hospitals.
The treaty covers battle tanks, armored combat vehicles, large-caliber artillery systems, combat aircraft, attack helicopters, warships, missiles and missile launchers, and small arms and light weapons.
The National Rifle Association, the most outspoken gun advocacy group in the US, opposes the treaty and strongly backs the Senate resolution that opposes it.
In a statement shortly after the UN vote, the resolution's sponsor, Republican Sen. Jerry Moran, said the treaty faces defeat in the Senate if President Barack Obama submits it for ratification.
"The passage of a treaty that Iran, Syria and North Korea have made clear they have no intention of abiding by will only serve to constrain law-abiding democracies like the United States," Moran said. "The US Senate is united in strong opposition to a treaty that puts us on level ground with dictatorships who abuse human rights and arms terrorists, but there is real concern that the administration feels pressured to sign a treaty that violates our constitutional rights."
Senate Foreign Relations Committee Chairman Bob Menendez, a Democrat, whose panel would hold hearings and the initial vote on the treaty, disputed such claims, arguing that the US negotiated a pact that "would in no way infringe on the rights of American citizens under domestic law or the Constitution to bear arms."
Menendez pointed out that Secretary of State John Kerry said the treaty only applies to international arms trade and does not affect an individual country's authority to enforce its laws on weapons.