The Facts

Although it is unclear what era in United States history Mr. Trump was referring to, it was most likely the period starting in the late 19th century, from after the Civil War to before World War I, said Douglas A. Irwin, a professor of economics at Dartmouth College.

During this period, the country maintained high taxes on imports that effectively blocked manufactured goods from foreign countries, Professor Irwin said. At the same time, the United States had high levels of economic growth and industrialization.

Some economists have cited a positive association between United States protectionism and economic growth during that era. In this high-tariff period, merchandise imports as a share of the country’s gross national product fell from 7.9 percent in 1869 to 4.2 percent in 1910, the historian Alfred E. Eckes Jr. wrote in his book about United States foreign trade.

The country emerged from this period with a larger share of world exports, wrote Mr. Eckes, who served as chairman of the United States International Trade Commission under President Ronald Reagan.