It would appear that the President is unilaterally withdrawing the United States from the 1972 ABM treaty without consulting Congress. Leaving aside the issues surrounding the pros and cons of the treaty itself, who is the President to say when we are going to withdraw from, or break a treaty? Another website brought this up, and the question has bothered me ever since.
The ABM Treaty does contain a clause that allows either of the signatories to withdraw after either 3 or 6 months notice, which is fine. It provides a way out. But who in our government gets to decide this? The Constitution says that while the President has the power to negotiate treaties, they must be approved by the Senate. Wouldn't this also imply that the Senate must also approve either breaking or ending a treaty arrangment? The Constitution doesn't say. I checked.
It appears to me that this is another instance of the President over-stepping his Constitutional authority (again), but I'm not an expert.