This is what I thought, but had to look it up to be sure: "Presently, there is no official Supreme Court ruling on whether the President has the power to break a treaty without the approval of Congress, and the courts also declined to interfere when President George W. Bush unilaterally withdrew the United States from the ABM Treaty in 2002, six months after giving the required notice of intent.[11]"
No matter what Trump is threatening now, he will come under a huge amount of pressure not to carry most of this out, if he gets elected.