In just a few days, a walkout by thousands of dock workers could bring commerce to a near standstill at every major US 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 wharf labourers are threatening to go on strike on Sunday - a wide-ranging work stoppage that would immediately close cargo ports on the country's east coast and the Gulf of Mexico to container ships.
The 15 ports involved in the labour dispute move more than 90 million tonnes of goods each year, or about 40 per cent of the country's containerised 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.
US 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 US 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 baulked at extending it again when it expires in the first minutes of 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 $US124,138 ($A120,180) 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 December 24.
As recently as December 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-containerised items like cars, and perishable commodities, like fresh food.
Yet the economic damage could still be severe.
"The global economy moves by water, and shutting down container ports along the East and Gulf coasts while the national economy remains fragile benefits no one," said Deborah Hadden, acting port director at Massport, the public agency that oversees shipping terminals in Boston.
It is not a part of the contract dispute.
Florida Governor Rick Scott said "the livelihood of thousands of Florida families lies in the balance".
The White House weighed in, too, urging dockworkers and shipping companies on Thursday to reach agreement "as quickly as possible" on a contract extension.
If it happens, the walkout could be the biggest US port disruption since 2002, when unionised dockworkers were locked out of 29 West Coast ports for 10 days because of a contract dispute.
The ports only reopened after President George W Bush, invoking powers given to him by the 1947 Taft-Hartley Act, ordered an 80-day cooling-off period.
Some economists estimated that each day of that lockout cost the US economy $US1 billion.
It took months for the retail supply chain to fully recover.