"After seven months of negotiations, we remain far apart on many issues," PMA spokesman Wade Gates said in a statement. "It is clear that the parties need outside assistance to bridge the substantial gap between us."
The union declined to comment on management's move except to say that it would issue a response on Tuesday. Both sides would have to consent in order for mediators to be brought into the negotiations, PMA spokesman Steve Getzug said.
Read MoreWest Coast port strife: The retail worry
The employers have repeatedly blamed the union for protracted shipping backups plaguing several West Coast ports since mid-October, most notably at the ports of Los Angeles and Long Beach, the nation's two busiest container cargo hubs.
Cargo that normally takes two to three days to flow through the affected ports, accounting for nearly half of U.S. maritime trade and over 70 percent of imports from Asia, have faced lag times of up to two weeks, according to industry analysts.
Port employers said the slowdowns could permanently undermine their business, citing an industry survey conducted by the Journal of Commerce reporting that 60 percent of shippers have begun rerouting 2015 cargos away from U.S. West Coast ports to avoid problems there.
While playing down the impacts as less dire than management has described, union officials point to other causes of the gridlock cited by officials of the ports themselves.
Chief among them is a chronic shortage of tractor-trailer chassis used for hauling cargo from ports to warehouses, a situation created when shippers sold off their chassis to third-party equipment-leasing companies.
The employers said no further talks were scheduled before the Christmas holiday.
The two sides opened their talks in May and mutually agreed to keep negotiating after their old contract expired June 30. The parties said in August they had reached a tentative deal on healthcare benefits but remained at odds over other issues ranging from wages to work rules.