Phil Gramm and Donald Boudreaux explain how new tariffs would blunt other positive aspects of President-elect Trump’s economic policy.

Donald Trump hopes to supercharge economic growth, restore manufacturing employment, and raise wages by imposing across-the-board tariffs of at least 10%, with even higher duties on Chinese goods. Yet any understanding of international trade, and of Mr. Trump’s first term, provides strong evidence that such measures wouldn’t achieve the president-elect’s objectives. The decline in economic growth caused by tariffs—along with reduced wages and income-tax collections—would at least partially offset any additional customs revenues. Implementing the tariffs would also likely trigger a trade war that would erode, if not overwhelm, the positive effects of tax reform and deregulation.

After Mr. Trump’s regulatory and tax relief boosted real economic growth from 1.8% in 2016 to a 13-year high of 3% in 2018, tariffs stunted growth. That was as the Congressional Budget Office predicted, with growth slowing to 2.6% in 2019, the first full year of the tariffs.

Employment in manufacturing continued falling as a percentage of total employment at the same rate as the previous decade. Before the tariffs were imposed, manufacturing jobs were 8.5% of total employment. The figure fell to 8.4% by the end of 2019 and 8.1% today.

Manufacturing output, after rising 2.5% during the first three quarters of 2018, fell when the tariffs fully kicked in. By the end of 2019, the inflation-adjusted value of manufacturing output was 6.2% lower than when the tariffs were imposed.

Mr. Trump’s pro-growth tax and regulatory policy attracted a surge of foreign investment, including from American companies operating abroad. Such investment required dollars, and the currency’s value soared. That made U.S. exports more expensive, imports cheaper and the trade deficit larger. The last was 24% larger when Mr. Trump left office than when he entered.

Imposing a 10% tariff on all imports would more than triple the average U.S. tariff rate on all imports, which in 2022 was 2.8%, slightly above the average tariff rate for Organization for Economic Cooperation and Development countries.