It undermines the balance of power and gives the president unconstitutional authority. Then I suppose you can show us some settled litigation proving Fast Track is unconstitutional. It's been 32 years since the legislation was passed. Certainly, during that length of time, someone or some organization has challenged this in court and proven it to be illegal.
Interestingly enough, the U.S. Supreme Court refused to hear the question in 2001, I believe. [I'd really like to thank all the protectionist/isolationist True Conservatives on this website, without whose never-ending canards my discovery of this case would have been impossible]. According to the
11th Circuit Court of Appeals, Fast-Track Authority is constitutional.