LOS ANGELES — The union representing the 930 clerical workers who handle much of the paperwork involved in loading and unloading ships at the ports of Los Angeles and Long Beach said yesterday they would strike if a contract agreement was not reached by midnight tonight.

If the threat is carried out, thousands of other unionized dockworkers could honor the picket lines. That would halt work at the ports — which handle more than 40 percent of the nation's containerized cargo — just as the heavy holiday shipping season for retail goods begins to gather steam.

Dock shutdowns in Los Angeles and Long Beach could result in a disruption in shipments to Hawai'i.

Late yesterday afternoon, talks continued between the office clerical unit of Local 63 of the International Longshore and Warehouse Union and 14 of the 17 ocean shipping lines and terminal operators with contracts under negotiation.

A spokesman for the larger 15,000-member union said that the clerical workers could expect their support.

"Longshore workers have always been there for any worker that is struggling for justice. That is part of the legacy and tradition of this union," said the spokesman, Craig Merrilees.

Representatives of both sides vowed to continue working for an agreement.

"We're still hopeful. No one is throwing in the towel," said Local 63 president John Fageaux Jr.

"We will keep at it. We are facing a deadline," said Stephen Berry, an attorney representing the 14 companies.

The negotiations have continued with few breaks since the workers' contract expired July 1. On June 29, the union's rank and file voted unanimously to give their leaders the right to strike if talks failed.

In the meantime, the ports' customers, which include some of the nation's biggest retailers, have become increasingly concerned about a shutdown and have been asking whether they should consider shipping their goods through other West Coast ports.

"We are saying that people should have their suppliers get ready to reroute and to expect an increase in costs," said Serkan Bayraktaroglu, import director for Dallas office of Commercial International Forwarding Inc.

Agreements had been reached on pension management and healthcare coverage. Berry said the companies were offering a generous package of salary increases and other benefits, but Fageaux said more concessions were needed.

Three years ago, both sides reached an 11th-hour agreement on a contract that also was negotiated under the threat of a strike and work at the ports continued uninterrupted.

Members of the office clerical unit handle the documentation and paperwork for the shipping containers moving into and out of the ports. Although called clerks, they are separate from the ILWU's marine clerks, who supervise the loading and unloading of cargo.

The ports of Los Angeles and Long Beach are by far the nation's largest container port complex, moving more than 15.8 million containers in 2006. More than 40 percent of the goods that come to U.S. shores in truck-size cargo containers flow through the twin ports on their way to destinations in every state, and 28 percent of exports from around the country leave through the local harbors.

That trade carried a value of more than $256 billion last year.