The senator may (incorrectly?) beleive that we would be passing control from the US to the UN, based in Geneva.
Then if someone in the UN makes a determination (say, that home-school building have to have t-bars and ramps or the disabled kid can't be home-schooled, even if he is visually imparied and doesn't need a ramp) the senator has no recourse within the US senate to amend that ruling.
Basically, he's losing control and doesn't like it. . .
P.S. I vote non-partisan.