A strike by dockworkers has shut ports from Maine to Texas that import more than half of U.S. container volumes—from bananas to BMW parts—and export everything from American soybeans to plastic pellets.

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.