The three most common suicide mechanisms for middle-age people are firearms, poisoning and suffocation, according to a CDC report. Reuters

Senate Republicans have long been wary of the United Nations. Now they are rallying in opposition to a UN treaty that would regulate the international arms trade, saying it would undercut Second Amendment rights.

Eight Democrats joined all but four Senate Republicans in signing a letter, drafted by Sen. Jerry Moran (R-Kan.), saying ratification of a treaty that fails to "protect ownership of civilian firearms" would fail in the Senate.

"Our country's sovereignty and the rights of American citizens must not be infringed upon by the United Nations," Moran said in a statement accompanying the letter, which was sent to President Obama and Secretary of State Hillary Clinton.

The arms treaty aims to regulate the international market for illicit small arms, a measure that proponents say will cut down on trafficking in illegal guns and the attendant violence. The United Nations has been working on the document throughout July and is set to release a final version.

Advocates point out that the Constitution overrides international law, but that has not mollified American gun rights groups who denounce the treaty as an infringement on American sovereignty.

"I am here to announce NRA's strong opposition to anti-freedom policies that disregard American citizens' right to self-defense. No foreign influence has jurisdiction over the freedoms our Founding Fathers guaranteed to us," National Rifle Association CEO Wayne LaPierre said at the UN Arms Trade Treaty Conference in New York earlier this month.

While the deadly shooting last week at an Aurora, Colorado movie theater has led gun control advocates to call for new safeguards, the reaction on the campaign trail has been relatively muted. Presumptive Republican nominee Mitt Romney has said new regulations would be unnecessary and President Obama has mostly stuck to emphasizing enforcement of existing laws, although he hinted on Wednesday at the need for an assault weapons ban he once supported.

The candidates have also avoided mentioning the U.N. treaty. But the issue has galvanized Second Amendment advocates and Tea Party groups who have long been suspicious of international organizations like the United Nations curtailing American independence.

Similar fears animated Senate opposition to the stalled Law of the Sea Treaty, a document that governs the use of sea lanes and underwater resources. Supporters have been unable to muster the two-thirds majority needed to pass the measure.

"No international organization owns the seas," Republican senators Kelly Ayotte and Rob Portman said in a recent letter to Majority Leader Harry Reid. "We are confident that our nation will continue to protect its navigational freedom, valid territorial claims and other maritime rights."