Exactly what part of the Constitution precludes Congress establishing up/down simple majority votes?
Article 2,Section 2, Clause 2: [The President] shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur
That part.
Fast Track should be unconstitutional. But the SCOTUS agreed that Congress can re-define trade agreements as separate from treaties, allowing them to write their own rules for approval.