I think Republicans have sensed the 'blood' here - that infighting among Democrats is going to inflame as the Trans Pacific Trade Deal moves forward. All these traditional Democrats, na, let us just call them 'legacy democrats'; identify themselves with the bygone era of Blue Color Workers whereas substantial votes in Democrats are coming from urban, cosmopolitan non-labor middle class. There is some merit in claims that these trade deals result in job losses. But then busting of Dot Com boom in 2000 and Great Recession in 2008 destroyed more jobs; not related to trade liberalization.
For Democrats to be able to explain all this to their all shrinking Blue Color base is to ask above their 'pay grade'. One bright spot on unionized labor is SEIU; I doubt how much of these service employees are impacted by trade liberalization. After all with open trade, Hotel Workers do not get laid off. Rather Hotels, Restaurants and larger Service Sector should thrive as trade gets liberalized.
From Republican perspective, what better deal can be there which makes life of Hillary Clinton difficult? With no intellectual over arching policy framework to explain why liberalized Trade should be avoided nor a consistent vision of how common Americans would alleviate their economic struggles in ever increasing inequality in this country; Democrats are simply not in a position to stop TPP. That is the opportunity GOP senses and the wheel of competitive politics turns a full circle here.