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.

Tens of thousands of dockworkers from ports spanning Maine to Texas walked off the job early Tuesday as they went on strike, demanding better wages and protections against automation. The strike comes as the contract between the International Longshoremen's Association union and U.