The nation’s retailers, manufacturers and farmers are bracing for a possible strike that could idle U.S. ports all along the Eastern Seaboard and Gulf Coast.

That walkout could begin as early as Sunday after the midnight Saturday expiration of an extension of a contract between the International Longshoremen’s Association and several shipping lines, terminal operators and port associations.

It would be the first strike by the ILA in 35 years. Negotiations on a proposed six-year contract broke down last week.

The biggest issue of contention involves so-called container royalty fees on cargo, which supplement dockworker wages. Employers want to cap those fees and limit who gets them.

The ILA says it represents a total of 65,000 dockworkers on the East and Gulf coasts as well as on several major U.S. rivers, on the Great Lakes, and in Puerto Rico and Eastern Canada.