Exactly where in the Constitution did you find that? As I stated, originally the Senate was to negotiate and make treaties, but before the Constitutional Convention ended, it was changed to say the President negotiates with the advise and consent of the Senate.
Left in its pure originally ratified form and intent, today the President would be negotiating treaties, but keeping the Senate informed at every step, asking if various amendments and wording would be a good idea, and accepting new wording. Consent would then be a given because the treaty would have been brought up to the Senate's liking in the first place.
However, through various precedents and Supreme Court rulings the role of the Senate has been legally reduced to that of yeah/nay with after-the-fact amendments. Given the progression, I can see the Supreme Court upholding Fast Track.
I'm not arguing whether Fast Track is a good thing or not -- it could be evil incarnate. What I am arguing is its Constitutionality.
Q, You need to remember that there is a difference between treaties and "ordinary run of the mill" laws. The problem is that DG, and others want to reverse the duties of the presidency and Congress in the making of LAWS, not treaties. It is an effort to parlaimentarianize the U.S. of A. form of representative constitutional government. If they were making these "agreements" as "treaties", there would be no constitutional question. But they are NOT! We should maybe ask, "WHY NOT??" Peace and love, George.