It is growing ever more likely that tens of thousands of dockworkers in contract negotiations will soon go on strike, snarling up supply chains and costing billions of dollars just weeks before the election.

A work stoppage at major East Coast and Gulf Coast ports may begin as soon as Oct. 1 if the U.S. Maritime Alliance doesn’t reach an agreement with the International Longshoremen’s Association. The existing contract expires on Sept. 30.