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If it happens, the walkout could be the biggest national port disruption since 2002, when unionized dockworkers were locked out of 29 West Coast ports for 10 days because of a contract dispute.

The ports only reopened after President George W. Bush, invoking powers given to him by the 1947 Taft-Hartley Act, ordered an 80-day cooling-off period. Some economists estimated that each day of that lockout cost the U.S. economy $1 billion. It took months for the retail supply chain to fully recover.