Fast track is clearly unconstitutional.
It's perfectly constitutional. All fast track does is establish a rule to be used for consideration of a trade deal negotiated by the executive branch. Congress still needs to vote to accept it in order for it to become law. Now if the trade deal would become law unless Congress voted to reject it, then that would be unconstitutional, but that is not what fast-track trade authority confers. All fast-track does is stipulate that when Congress considers the deal the executive has negotiated, it will give it a simple up or down vote without any amendments being considered.
He shall have Power, by and with the Advice and Consent of the Senate, to make Treaties,
provided two thirds of the Senators present concur;
