If the investors know what is in it they will try to preposition themselves. If the union knows what's in it and don't like it they will try to pick it apart.
NAFTA was done in complete secrecy. Congress didn't know what was in it until it was signed in Oct 2002 by the trade representatives. And it was not known who would win the presidency.
Many in Congress Thought that the investor protections were to strong and the protection for labor and environment were lacking, but they can't change it. They can only approve it or kill it.
They didn't want to kill it because so much time was put into it. So when Clinton won the presidency they all decided that Clinton would negotiate side agreements which would contain the protections for labor and environment, which he did in 1993. Those were all ratified and went into force on Jan 1, 1994.
But these side agreements that Clinton negotiated turned out to be toothless because the arbitration panel refused to honor them because they weren't written into NAFTA.
So when GW Bush began negotiating CAFTA he would be transparent and not negotiate any term that would kill the deal in Congress. And CAFTA was easily ratified.
In the case of Obama and TPP, the GOP knew what was in it before they gave Obama fast track authority, except they didn't know absolutely everything because some of it is decided by negotiations. From the investors perspective there are few objection to TPP. The drug companies are complaining.
OTOH, the unions and enviros are absolutely sure that Obama is cutting their throats. And they know that Hillary is lying when she says that she objects to it. This is why Maureen Dowd proclaimed Hillary to be the perfect GOP candidate for the presidency.
Next argument?