Attempting to understand Trans Pacific Partnership

For the next few weeks, I am changing course in writing about environmental issues in agriculture and will attempt to provide some background on the Trans Pacific Partnership, a trade agreement among 12 Pacific Rim countries of Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore, United States and Viet Nam. The proposed TPP was signed on February 4, 2016, in Auckland, New Zealand. It took seven years of negotiations and needs to be ratified within two years after initial signatures. Proponents claim 30 chapters of the TPP will enhance economic growth, help support and create new jobs, raise living standards, and enhance labor and environmental protections.  Many voices in agriculture support TPP, but presidential candidates oppose it. Why?