The President has a lot of discretion, but there are limits.
At least that seems to be the point made in the FP article Giving Away The Farm.
Authors James Woolsey and Rebeccah Heinrichs suggest the President’s arms controllers—who would be operating under his direction—may have pledged something beyond a simple change to the nuclear weapons count. They wonder out loud if part of the enticement to the Russians to get them to commit to New START may have been to promise to share sensitive U.S. technologies, methods, information, and even operational control on missile defense.
Offered as plausible speculation is the President’s threat to veto the defense budget over Congressional language which “requires that no funds can be used to provide the Russian Federation with sensitive U.S. missile defense technology.”
You might think that the House doesn’t need to insert such language, but there it is. The language is included perhaps because the negotiating records/audit trail that supported New START has not (to my knowledge) been released to the Congress. As such, concerns remain and the restrictive language is part of the draft legislation.
By the way, why would the President threaten to veto the bill over what’s really pretty benign (and which should be unnecessary) language which restricts the Russians from getting our expensive, important, and sensitive missile defense products?
If the authors are correct, this would all also seem to serve as a confirmation that (contrary to some of the nutbags posting in the comments to the article) missile defense works and that it presents a significant Russian national security concern.