Opposition to the TPP isn't as much about Clinton as it is about it being perceived as a rebuke of President Obama, who we all should remember is the guy negotiating the thing. He's staked his reputation on it and nobody in Democratic leadership wants the party platform to so soundly rebuke the sitting president, especially when TPP had significant support among Democrats in Congress.
But to what degree would it be convenient for the party to drop support for TPP as long as they don't need to take personal responsibility for that happening? I just wonder whether there might be a sense that they can use popular pressure as an excuse for shifting the party to avoid conflict or distrust among the people who are leading it.
As far as I know the platform isn't supporting the TPP so much as not opposing it, essentially leaving it up to the congresscritters to decide how they will vote. Of course even if opposition makes it into the platform nobody's mind will be changed and the platform isn't binding on anyone, so it is essentially just a shot across the bow towards Obama.
