The 15 ports involved in the labor dispute move more than 100 million tons of goods each year, or about 40 percent of the nation's containerized cargo traffic.

NEW YORK — More than 14,000 longshore workers are threating to go on strike Sunday, a work stoppage that would immediately close cargo ports on the East Coast and in the Gulf of Mexico to container ships.

The 15 ports in the labor dispute move more than 100 million tons of goods each year, or about 40 percent of the nation’s containerized cargo traffic. Losing them to a shutdown, even for a few days, could cost the economy billions of dollars.

Shipments of such varied products as flat-screen TVs, sneakers and snow shovels would either sit idle at sea or get rerouted, at great time and expense. U.S. factories also rely on container ships for parts and raw materials, meaning supply lines could be squeezed.

The master contract between the International Longshoremen’s Association and the U.S. Maritime Alliance, a group representing shipping lines, terminal operators and port associations, expired in September. The two sides agreed to extend it once already, for 90 days, but they have balked at extending it again when it expires at 12:01 a.m. Sunday.

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The work stoppage would not be absolute. Longshore workers would continue to handle military cargo, mail, passenger ships, noncontainerized items such as automobiles, and perishable commodities, such as fresh food.