"It took the ports a full six months to recover, and the strike had a profound impact on the retailers, importers, manufacturers, agricultural exporters and other affiliated industries that rely on the ports every day," Shay's letter continued. "An extended strike this time could have a greater impact considering the fragile state of the U.S. economy."
The NRF called on Obama to "use all means necessary to get the two sides back to the negotiating table" because it is "critically important ... that the two parties remain at the table until a new deal is finalized."
The ILWU's Local 63 union's Office Clerical Unit (OCU) that is representing southern California harbor employees has established picket lines at the ports of Los Angeles and Long Beach.
The parent ILWU 63 group said it was supporting the workers' strike because "[A]n injury to one is an injury to all."
"We thank the membership for the solidarity and support shown while our OCU brothers and sisters fight for a contract," the union said on its website.
The Los Angeles/Long Beach Harbor Employers Association said on Thursday that the work ILWU's work stoppage had shut down 10 of the 14 terminals in the Los Angeles and Long Beach ports.
The employers association called the strike "uncompromising and disruptive tactics" that "run counter to the best interests of the Los Angeles region and the nation" in a statement on its website.
"In an effort to resolve this dispute, the harbor employers have offered to enter into mediation on numerous occasions throughout the last two and one-half years, as recently as this week," the Harbor Employers Association statement said. "However, the OCU rejected the offer and put up pickets instead."
The NRF said the negotiations between the two sides are "important to all of the import and export industries who rely on the nation’s largest ports to move the nation’s commerce." He added that "we cannot afford further supply chain disruptions."
Shay expressed confidence the negotiators could reach a deal if they continued talking to each, however.
"We fully believe the two sides can reach an agreement that will ensure the continued competitiveness of these ports for the foreseeable future," he said.
A similar labor dispute threatened ports on the East Coast earlier this fall, but negotiators in that case agreed on an extension of the facilities' existing collective bargaining agreement through the end of December to prevent a holiday work stoppage.