Trump, in advocating for Congress to support his new trade agreement, said he had met men and women across the country whose "dreams were shattered by NAFTA." 

There are many estimates for job losses due to the Clinton-era North American Free Trade Agreement (NAFTA), but “millions” is higher than the number calculated by most groups from around the ideological spectrum.

Robert E. Scott of the pro-labor Economic Policy Institute found that about 851,700 U.S. jobs were displaced by the U.S. trade deficit with Mexico between 1993 (shortly before NAFTA was implemented) and 2014. That’s a data point that was cited by Bernie Sanders during his 2016 campaign, when he frequently decried job losses due to NAFTA.

A 2014 Peterson Institute for International Economics study found that while NAFTA has caused about 203,000 jobs to be displaced by NAFTA-related imports annually, imports support 188,000 new jobs, leading to a net loss of only about 15,000 annually.

And the nonpartisan Congressional Research Service wrote in 2017 that “in reality, NAFTA did not cause the huge job losses feared by the critics or the large economic gains predicted by supporters. The net overall effect of NAFTA on the U.S. economy appears to have been relatively modest, primarily because trade with Canada and Mexico accounts for a small percentage of U.S. GDP.”