Here’s a lesson in America’s weird political institutions: How Christian conservatives lead the Republican party to reject a treaty that endorsed existing American law.

The US Senate voted today on ratifying the UN Convention on the Rights of Disabled People 61-38, but the majority fell short of the 66 votes needed for ratification. The 38 votes against came from Republican senators, most of whom signed a letter promising not to support the bill. The letter was organized by Senator Mike Lee of Utah, who said the treaty threatened US sovereignty and could force the parents of disabled children to send them to public schools. It drew the support of home-schoolers who also fretted that the treaty was, among other things, a sly way to force America to adopt laws enshrining “abortion rights, homosexual rights, and demands the complete disarmament of all people.”

The last reference, obviously, is designed to scare American conservatives into believing that ratifying the treaty will empower the US government to take away their right to bear arms, enshrined in the American constitution. But the interpretation relies on the assumption that vague, utopian boilerplate about disarmament in an entirely different UN convention will somehow be applied if the US ratifies the convention on disability. Nevertheless, this sort of stuff, promoted by evangelical politicians like former Senator Rick Santorum, has a galvanizing effect on Republican politicians who live in fear of losing intra-party challenges from the right. It is the preserve of an isolationist wing of a party already deeply skeptical of international institutions generally.

But aside from how overblown such fears are, the stranger fact is that the UN treaty is based on the Americans with Disabilities Act, enacted 22 years ago, and if it were ratified, no US laws would have to change. It was negotiated by the previous Republican president, George W. Bush, and is supported by prominent conservatives like Senator John McCain and former Senate majority leader Bob Dole (both of whom, thanks to war wounds, are Americans with disabilities). The US Chamber of Commerce supported the treaty, since it would help level the international playing field for American companies who already comply with the act, and potentially open foreign markets to US disabilities technology.

The treaty’s critics, like the conservative Heritage Foundation, were left arguing that the treaty shouldn’t be ratified if the US already complied with its intent, since endorsing the treaty could lead to problems down the road by unspecified means. That dismayed the treaty’s advocates, who see the treaty’s value in the message it sends to other countries about the importance of protecting disabled people. “It’s a treaty to change the world to be more like America,” protested John Kerry, the Democratic Chairman of the Senate Foreign Relations Committee, before the failed vote.

If you’re watching the US Senate for signs of how the fiscal cliff debate will play out, take note. This is what a dedicated extremist minority can do when the consequences of a decision are incredibly anodyne. Imagine what could happen when real questions of tax and social insurance structures are on the table in the weeks ahead.