While many analysts focus on the number of jobs lost from Nafta and similar pacts — and some estimates say upward of a million — the most significant effect has been a fundamental change in the composition of jobs available to the 63 percent of American workers without a college degree.

At Nafta’s core — and proposed for the T.P.P. — are investor rights and privileges that eliminate many of the risks that make firms think twice about moving production to low-wage countries. Today, goods once made here are being produced in Mexico and exported here for sale. Indeed, American manufacturing exports to Mexico and Canada grew at less than half the rate after Nafta than in the years before it.

As a result, our trade deficit has ballooned. In 1993, before Nafta, the United States had a $2.5 billion trade surplus with Mexico and a $29 billion deficit with Canada. In 2012, the combined Nafta trade deficit was $181 billion, even as the share of that deficit made up of oil imports dropped 22 percent. The average annual growth of our trade deficit has been 45 percent higher with Mexico and Canada than with countries that are not party to a Nafta-style pact. The companies that took the most advantage of Nafta — big manufacturers like G.E., Caterpillar and Chrysler — promised they would create more jobs at their American factories if Nafta passed. Instead, they fired American workers and shifted production to Mexico.

The Labor Department’s Trade Adjustment Assistance program, which documents this trend, reads like a funeral program for the middle class. More than 845,000 workers have been certified under this one narrow and hard-to-qualify-for program as having lost their jobs because of offshoring of factories to, and growing imports from, Mexico and Canada since Nafta.

The result is downward pressure on middle-class wages as manufacturing workers are forced to compete with imports made by poorly paid workers abroad. According to the Bureau of Labor Statistics, nearly two out of every three displaced manufacturing workers who were rehired in 2012 saw wage reductions, most losing more than 20 percent.