Gordon Hanson, an economics professor at the University of California, San Diego, emailed me his analysis of Trump’s economic scheme:

Trump’s strategy is essentially one of withdrawal from the world economy. He wants less trade and less outward foreign investment. He offers no plans for how to improve our export performance. This is protectionism, pure and simple.

Erik Brynjolfsson, an economist at M.I.T.’s Sloan School of Management, was more forceful:

No nation can succeed by trying to protect the past from the future. We will succeed by having the confidence to embrace competition, and leveraging our comparative strengths, which are numerous. We have the largest, most productive and most technologically advanced economy that’s ever existed on this planet. The more open the world economy is, the more we have an opportunity to leverage our many strengths.

Looked at this way, Trump’s stance is an implicit admission that he and his followers do not “believe in America” — an argument that the United States cannot compete successfully in the world arena unless protected by the imposition of high tariffs and punitive taxes on foreign production and foreign competitors.

Robert Reich, an economist at Berkeley, former secretary of labor under Bill Clinton and a leading supporter of Bernie Sanders during the primaries, agrees.

Trump’s trade proposals, Reich argues,

assume the U.S. can’t compete and must erect trade barriers lest other countries flood America with better and cheaper products. That’s the opposite of believing in America.

On Jan. 7, Trump told The New York Times that he would impose a 45 percent tax on goods imported from China. “I would tax China on products coming in,” he told the paper’s editorial board. “The tax should be 45 percent.”

When Ford proposed building new manufacturing facilities in Mexico, Trump declared in a September 2015 speech that he would call the president of Ford and tell him:

I don’t want you to do that. And if you do it, you’re not going to have any cars coming across the border unless you pay a 35 percent tax.

Trump said the same thing in March after the Carrier Corporation announced plans to move air-conditioning production facilities to Mexico:

I’m going to tell the head of Carrier: “I hope you enjoy your stay in Mexico folks. But every single unit that you make and send across our border, which now will be real, you’re going to pay a 35 percent tax.”

Andrew McAfee, a director of M.I.T.’s Initiative on the Digital Economy and co-author of “The Second Machine Age: Work, Progress, and Prosperity in a Time of Brilliant Technologies,” was sharply critical of Trump. In an email, McAfee wrote:

There’s a reason that all good economists support free trade, and that none of them are supporting Trump’s proposals. The reason is that trade gives us more and better access to goods and services than we could produce on our own. It also provides jobs for exporters, people working in airports and ports, and so on. Free trade is not surrender, and not something that only suckers do. In fact, just the opposite. Closing our borders would be surrender to a nonexistent enemy. It would make us poorer without bringing back the jobs.

Sean Wilentz, a historian at Princeton, contends that Trump’s proposal is only slightly less drastic than the Smoot-Hawley Tariff — a law passed over the objection of more than 1,000 economists and signed by Herbert Hoover in June 1930. Smoot-Hawley is largely acknowledged as one of the principle causes of the subsequent worldwide economic catastrophe. In an email, Wilentz wrote:

Smoot-Hawley raised tariffs across the board, with every trading partner. The results were disastrous for the world economy, let alone for the U.S. at the opening stage of the Great Depression. A worldwide trade war commenced, and international trade was shattered. Trump so far has proposed only sharp tariff hikes with Mexico and China — but these are two of the three largest sources of U.S. imports.

If the tariffs on goods produced in Mexico and China were enacted, the results, in Wilentz’s view, would be “pretty severe. Right away, U.S. domestic prices would rise significantly unless and until other sources take up the slack. And then comes the Mexican and Chinese response, killing exports to our second and third largest customers, respectively.” (Canada is No. 1.)

Many economists share the view that Trump’s trade proposals would be ruinous to the American economy, but in order to retain union support, Hillary Clinton has not been able to directly challenge Trump on these grounds. It was Sanders’s primary campaign that prompted Clinton to abandon past support for free trade agreements.