NEW YORK – In just a few days, a walkout by thousands of dock workers could bring commerce to a near standstill at every major port from Boston to Houston, potentially delivering a big blow to retailers and manufacturers still struggling to find their footing in a weak economy.

More than 14,000 longshoremen are threatening to go on strike Sunday – a wide-ranging work stoppage that would immediately close cargo ports on the East Coast and the Gulf of Mexico to container ships.

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. Losing them to a shutdown, even for a few days, could cost the economy billions of dollars.

“If the port shuts down, nothing moves in or out,” said Jonathan Gold, vice president of supply chain and customs policy at the National Retail Federation. And when the workers do return, “it’s going to take time to clear out that backlog, and we don’t know how long that it’s going to take.”

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 for all sorts of products could be squeezed.

Joseph Ahlstrom, a professor at the State University of New York’s Maritime College and a former cargo ship captain, called container ships the “lifeblood of the country.”

“We don’t fly in a lot of products. It’s just too expensive,” Ahlstrom said. “The bulk of the products we import come in inside containers.”

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 so far balked at extending it again when it expires at 12:01 a.m. Sunday.

The union said its members would agree to an extension only if the Maritime Alliance dropped a proposal to freeze the royalties workers get for every container they unload. The Alliance has argued that the longshoremen, who it said earn an average $124,138 per year in wages and benefits, are compensated well enough already.

Federal mediators have been trying to push negotiations along, but there has been no word from either side on the progress of the talks since Dec. 24. As recently as Dec. 19, the president of the longshoremen, Harold Daggett, said the talks weren’t going well and that a strike was expected.

The work stoppage would not be absolute. Longshoremen would continue to handle military cargo, mail, passenger ships, non-containerized items like automobiles, and perishable commodities, like fresh food.

Yet the economic damage could still be severe.