Trade is not racially neutral "economic populism." When Sanders talked about trade, it was to talk about how corporations use trade to harm workers and democracy at home and abroad. When Trump talks about trade, he talks about other countries "stealing" jobs from "the United States"-- a zero sum image that implicitly argues for Trump taking back jobs from brown people and giving them back to his white constituency.
The liberal support for free trade in the immediate post-World War II period was that it was replacing "managed trade" of Europe with its global partners, better known as colonialism, where those with military and economic power would negotiate individual deals with weaker clients and extract their wealth to the advantage of the main European power's citizens and workers. Even as workers might be fighting capitalists at home, they might be sharing in the exploitation of poor Asian or African or Latin American workers -- essentially a global Jim Crow of white privilege for workers in the Anglo-American world.
Creating multi-lateral rules where every nation would follow the same rules with no nation getting special privileges or benefits in any trading relationship would ideally limit that colonial extraction of surplus. Given global imbalances of wealth and power, that was always more an ideal than complete reality but for decades it seemed better than colonial system it replaced.
But Trump is de facto calling for a return to the colonial era of individual negotiations with countries like China or Japan, or inevitably whatever non-white nation he rails against, where the U.S. can use its military and economic power in the hands of master negotiator to extract a "better deal" from each individual nation to benefit its own citizens at the expense of the poorer, less white people of those other countries.
This is quite different from the progressive critique of modern trade. Where progressives argue things went wrong with trade deals of recent decades was that they began focusing less on lowering tariffs and making trade equal and more on protecting individual corporate interests, whether expanding intellectual property rules or restricting local consumer and environmental protection rules or trying to give corporations a legal right to strike down laws they don't like in unelected arbitration panels. They failed to strengthen labor laws at home and abroad to ensure that lower priced goods in a country didn't just reflect more exploitation.
Trump never rails against trade deals being used by our companies to hurt U.S. workers, just as his overall policy doesn't argue for strengthening labor laws or taking any other action at the expense of the corporate elite on behalf of his white working class constituency. No, the only groups he argues should lose economically to enhance the wealth of that constituency are poor immigrants at home and poor non-white workers abroad.
Trump's trade "populism" is as much a racist appeal to his voters as his attacks on immigrants or his denunciations of muslims. It's all part of othering non-white people to justify policy to advantage that white constituency over other groups. That those white workers in the U.S. have legitimate gripes with how the global economy may be treating them doesn't make Trump's appeals any less racist.