We do not have an effective Intercontinental Ballistic Missile Defense system, and no one has ever demonstrated that such defense is technologically and strategically workable. This point should have been brought home on the day the Senate voted to begin debate on the treaty. That same day our existing Missile Defense system was tested, and once again, for the second time that year, it failed.
And it's not just practical considerations. Think about missile defense strategically and it's still a terrible idea:
Some ABM advocates have argued that even if strategic missile defense systems have fundamental technological obstacles, simply the threat of a system that might shoot down some incoming missiles is enough to dissuade a possible aggressor from attacking. Logic suggests otherwise. In the first place, if an attack was based on rational decision-making (and again, since such an attack would have a high likelihood of being followed by an annihilating counterattack it is hard to wonder how reason would enter into such a decision)—presumably to inflict damage or terrorize our country—then in the face of an imperfect ABM system, reason would dictate launching several missiles instead of 1 against any prospective target.
To recap: missile defense is a strategically dubious program that doesn't actually work. Oh, and we've spent over $100bn on it. We're facing a couple trillion dollars in needed infrastructure repairs and upgrades that nobody wants to do anything about. But we can spend $100bn on missile defense. Fiscal responsibility, my ass.