The United States imported $349.6 billion of goods from Mexico in 2018, and it sent $265 billion back across the border, according to statistics from the Union Nations’ International Trade Center, making Mexico the United States’ largest trading partner after China and Canada.

Automakers, who have spread their supply chains across North America, would be among those most affected. Shutting the border would lead to sudden disruptions in the supply chains for auto parts, causing car production lines in South Carolina, Michigan, Indiana and Alabama to shutter. The change would begin to affect American auto production in less than one week, said Ann Wilson, the senior vice president for government affairs at the Motor & Equipment Manufacturers Association, which represents auto parts makers.

Other industries would be hard-hit as well. Many manufacturers of flat-screen TVs, computer hardware and medical devices are set up south of the border, while the apparel industry sources raw materials like cotton, buttons, zippers and threads from the United States and sews them into finished products in Mexico.

Closings would also affect sectors outside manufacturing, including energy — the United States supplies much of Mexico’s natural gas through pipelines that run across the border — as well as the transportation sector, where hundreds of thousands of American jobs are tied to moving goods to and from Mexico through ports and by truck or train.

Over all, some of the world’s biggest multinational companies would be affected — including Ford, Toyota, Boeing and Caterpillar — but so would countless small- and medium-size businesses that depend on trade to make and sell their products.

Grocery shoppers and farmers

The signing of Nafta prompted a huge reorganization in agriculture on both sides of the border. Now, consumers in the United States depend on Mexican farmers for fresh fruit and vegetables like avocados, tomatoes, strawberries, grapes and mangoes, as well as packaged foods and beer. Mexico relies heavily on American soybeans, corn, dairy products, chickens, beef and other goods.