NEWARK, N.J. – A union representing dock workers at the East Coast’s busiest port has authorized a strike if a contract deal isn’t reached by the end of next month, lending urgency to preparations by retailers to send cargo elsewhere if labor talks affecting the entire seaboard remain at a standstill.

The negotiations affect ports up and down the East Coast and turn on key issues of overtime rules and container royalties, which are payments to union workers based on the weight of cargo received at each port. Talks broke down last week, and both sides said Wednesday no new discussions had been scheduled.

Some retailers had already put contingency plans into action and were rerouting ships to the West Coast or seeking other alternatives, while others were on the verge of acting, according to Jonathan Gold, vice president of supply chain and customs policy for the National Retail Federation.

“Some already have decided to move cargo to the West Coast or to bring it earlier to the East Coast,” Gold said. “From what most folks have told me, they’re going to decide in the next week whether they’re going to pull the trigger.”

The Port of Los Angeles was prepared for the additional traffic, spokesman Phillip Sanfield said. Any rerouted ships likely won’t arrive for a few weeks, he said.

A spokesman for Local 1804-1 of the International Longshoremen’s Association confirmed the union’s strike authorization vote was entered late Tuesday. The local represents about 1,200 of the Port of New York and Jersey’s 3,500 longshoremen.

McNamara accused the alliance of cherry-picking numbers and focusing only on a small minority of highly paid employees. He said the container royalties serve a valuable purpose by defraying benefits costs for union members in smaller ports, such as New Orleans and Jacksonville, Fla.

“Up until last Wednesday, we thought we were very close,” McNamara said. “Then they came in with a very hardline stance all of a sudden, and that resulted in the two sides as being very far apart.”

A strike could be economically devastating for Savannah, Ga., the second-busiest container port on the East Coast.