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.

The walkout threatened to plunge the holiday shopping season into turmoil and become an issue in next month's presidential election.

A strike by dockworkers at dozens of U.S. ports has paralyzed about half of the country’s ocean shipping and a prolonged dispute could bring the issue close to home for consumers. The strike, which included about 45,