The fear here — and not just limited to the disabilities treaty — is that international law can be used to override U.S. law. There are, in fact, cases in which litigious activists have tried to enforce international law in U.S. courts. But this treaty — and the congressional Reservations, Understandings and Declarations (RUDs) that accompany it — provides no basis for such challenges.
The resolution of ratification makes clear that the Americans with Disabilities Act remains the controlling U.S. law and that the treaty will not affect current enforcement or create additional causes for legal action. The treaty cannot be enforced through U.S. courts, and it creates no international venues for litigation. Recommendations from the committee of experts are non-binding. This does not require trusting the United Nations, only trusting the supremacy of the Constitution and U.S. law.
So if the treaty leaves U.S. law exactly the same, why take the risk of signing at all? The answer depends on your broader view of America’s calling. If you think American exceptionalism is sullied or stained or corrupted by engagement with the world, then isolation from the contagion of international norms is a natural response.