Why don’t Democratic members of Congress support free trade agreements? The reason most often given is that historically Democrats have been the party of the working person. And since the conventional wisdom holds that trade destroys jobs in the U.S., Democrats have to oppose it.
Related to this explanation is that labor unions are de facto extensions of the party and if they oppose trade then the party’s elected representatives must follow their lead on this issue. Since unions donate money to candidates they support, to favor trade means saying goodbye to these contributions and the votes they represent.