The union representing U.S. dockworkers has signaled that 45,000 members will walk off the job at midnight, kicking off a massive strike likely to shut down ports across the East and Gulf coasts.

Negotiations between longshoremen and port operators broke down in June and the two sides have barely been speaking. The current contract expires at midnight Monday.

Port workers at ports from Maine to Texas began walking picket lines early Tuesday in a strike over wages and automation that could reignite inflation and cause shortages of goods if it goes on more than a few weeks.