Opponents say not testing will make the American arsenal unreliable. But the United States is already not testing. Moreover, no world leader is going to be less afraid of American nuclear power because we don't test. Opponents also say the treaty can't be verified. It can't, with total certainty.
Few treaties can.
But major and important violations are likely to found, and if they are, Washington would no longer be bound by the treaty. The risks in ratification were so small. So were the motives in rejection.