Protectionism will not add to American jobs or raise wages, on the whole. At best it will shuffle jobs around — adding some in protected companies like steel makers and cutting some in industries that buy steel, like auto manufacturers. By making the economy less efficient, protectionism will also make the nation poorer as a whole.

But maximizing economic output is not the nation’s only objective. The case for trade liberalization also relies on the proposition that winners in the process will compensate the losers whose jobs are displaced. If American politics impedes any redistribution of trade’s spoils, perhaps there is a case for restoring equity by throwing sand in the cogs of trade.

“It would be decreasing the size of the pie to increase the size of some slices,” as David Autor, a top labor economist at the Massachusetts Institute of Technology, put it to me once. “We have always done the opposite thing without making people whole.”

What’s more, unanticipated shocks over the last quarter-century — when information technology swept through every industry and China emerged from nowhere to become the world’s biggest exporter — might justify reconsidering market-access commitments made earlier. “Maybe at the end of the day some trade responses are reasonable,” said Robert W. Staiger, a trade economist from Dartmouth College.

And yet even accepting that the United States may find it reasonable to retreat from trade somewhat, it is critical to figure out what this retreat might look like.

The many rounds of trade liberalization after World War II were anchored in two core principles that, in fact, had been adopted by the United States in 1934: reciprocity and nondiscrimination. Countries could expect to receive concessions as valuable as those they offered. Most critically, a concession made to one country would automatically be extended to all, under what was called the most-favored-nation rule.

The cocktail worked. Notably, the principle of nondiscrimination ensured that a given trading partner could not negotiate a tariff cut with the United States and then offer a more favorable deal to another country — undercutting the American competitive position. This broke through a logjam that had stymied previous attempts to liberalize international trade by encouraging countries to make only miserly offers.