The populist arguments against NAFTA have generally been that it "enriches corporations, at the expense of American jobs". While it eased Canadian-US trade somewhat, the most visible effect of NAFTA was that US-Mexican trade was eased to the point that hundreds of maquiladoras (manufacturing facilities) sprung up close to the US border. Among other changes, Mexico has now become a top-10 exporter of automobiles.
The maquiladoras have enhanced the lives of many millions of Mexicans. Meanwhile, it had a mixed effect on the USA, in particular pressuring hundreds of thousands of US autoworkers. Benefits to the US were much more diffuse than the lost autoworker jobs, leading many people to conclude those benefits were negligible. That's a common policy-maker's problem, where a special-interest group (here, US autoworkers) holds policy or public opinion hostage to its interests because the incremental advantage of good policy is, while larger in aggregate, thinly spread among a large constituency. It's quite recognizable in, for example, the activities of the sugar lobby on influencing congressional lawmakers.
Such lobbies, by the way, are a big reason trade agreements must be negotiated privately, keeping details hidden from the public. Otherwise, special interest groups end up completely destroying the process while negotiations are underway. Remember, sugar tariffs are very good for the sugar lobby.
While I appreciate patriotism, I personally feel that we should be trying to make life better for humanity in general, rather than greedily holding onto wealth in the USA. Taking at face value the Wharton study quoted above, the USA was able to enrich Mexicans at zero cost to itself. From that point of view, similar trade agreements are nearly a moral imperative!
Coming back to TPP, it has some leaked aspects that I think are truly terrible, such as the intellectual freedom troubles. Those criticisms I consider reasonable, and I can appreciate why that would cause an informed and intelligent person to oppose the TPP. On the other hand, a kind of knee-jerk hatred to trade agreements in general appears to drive much of the opposition, and I think of those anti-trade arguments as having no moral standing, just like the ones put forth by the sugar lobby.
On balance, then, I think the benefits to human happiness worldwide from even an agreement with flawed and overly-broad terms will outweigh the serious problems, but I can see how intellectual freedom considerations might make you feel otherwise.