When a dockworkers’ union broke off contract talks with management in June, raising the likelihood of a strike at more than a dozen ports on the East and Gulf Coasts that could severely disrupt the supply chain this fall, it was not over wages, pensions or working conditions. It was about a gate through which trucks enter a small port in Mobile, Ala.

The International Longshoremen’s Association, which has more than 47,000 members, said it had discovered that the gate was using technology to check and let in trucks without union workers, which it said violated its labor contract.

“We will never allow automation to come into our union and try to put us out of work as long as I’m alive,” said Harold J. Daggett, the union’s president and chief negotiator in talks with the United States Maritime Alliance, a group of companies that move cargo at ports.

The I.L.A., which represents workers at economically crucial ports in New Jersey, Virginia, Georgia and Texas, has long resisted automation because it can lead to job losses.

Longshoremen have grim memories of how past innovation reduced employment at the docks. Shipping containers, introduced in the 1960s, allowed ports to move goods with fewer workers. “You don’t have to pay pensions to robots,” said Brian Jones, 73, a foreman at the Port of Philadelphia. He began working there in 1974, when bananas from Costa Rica were unloaded box by box.

Workers throughout the economy are worried that technology will eliminate their jobs, but at the ports it threatens one of the few blue-collar jobs that can pay more than $100,000. The United States has done less to automate port operations than countries like China, the Netherlands and Singapore. But the technology is now advancing more quickly, especially on the West Coast.