LONG BEACH, Calif. -- Rafael Rivera spends more than a dozen hours a day in the cab of his old blue truck. In a typical day spent hauling shipping containers from the Port of Long Beach to distribution centers for companies such as Ikea, Pier 1 Imports or Toyota, the El Salvadoran immigrant clears about $8 an hour.

Mr. Rivera's humble but critical job is about to come under scrutiny. A federal antiterrorism program set to go into practice later this year will require port workers -- including more than 100,000 drivers like Mr. Rivera who pass through the nation's terminals -- to prove they are legal U.S. residents. While Mr. Rivera says he's here on a work permit, many drivers in this predominantly Hispanic work force are believed to be illegal immigrants. For the harbor trucking industry, which already faces a shortage of drivers, the new regulations promise to make the crunch acute.

Washington's bid to ratchet up port security is expected to leave thousands of undocumented drivers without jobs in ports across the country. That would undermine the flow of goods to U.S. consumers, highlighting the crucial role immigrants such as Mr. Rivera play in the nation's economy.

At issue is the Transportation Worker Identification Credential program, or TWIC, a component of the Maritime Transportation Security Act of 2002. Under this federal program's guidelines, workers with unescorted access to secure U.S. port areas must be legal residents and pass a background check. Disqualifying felonies are likely to include robbery and drug dealing. Those who pass will be issued biometric identification cards, which include a photo and fingerprints. Workers will gain access to a secure port area by swiping the cards at the entrance.

The worker-credential plan wasn't crafted with illegal immigrant workers in mind. It was an outgrowth of the Sept. 11, 2001, terrorist attacks, which alerted authorities to the vulnerability of U.S. infrastructure and raised concerns that radioactive materials could be smuggled into one of the country's 180 commercial ports. After languishing for years, TWIC -- which will be implemented by two Homeland Security divisions, the U.S. Coast Guard and the Transportation Security Administration -- is expected to launch late this year, says a TSA spokesman.

The program will require identification cards for about 750,000 port workers nationwide, which include about 110,000 truck drivers, the TSA says. A large proportion of the truckers are Hispanic -- as much as 90 percent at the sprawling, adjacent harbors of Los Angeles and Long Beach, the nation's busiest ports. No one knows for sure how many are illegal immigrants, though estimates range from 20 percent to 50 percent nationally. With these truckers out of the work force, major ports such as Houston, Miami and New York-New Jersey would also be heavily affected.

"We need TWIC for security," says Bill Madden, general manager for terminal services at Long Beach Container Terminal Inc. "But it will cause a big dislocation. The truckers are the guys who make this place work."

The drivers form a critical link between global suppliers to retailers throughout the U.S. Nationwide, they haul everything from spare auto parts and life-saving medical devices to toys and T-shirts from harbors to distribution points 50 or 100 miles away. Working as independent contractors, they are paid a flat per-trip fee -- typically $40 to $100, depending on the distance between port and warehouse, and averaging about $55. Most drive aged trucks -- known as bobtails -- to which they attach trailer rigs. They normally take out loans to buy the trucks, paying them off in monthly installments. The drivers are paid by trucking firms retained by freight forwarders, steamship lines and importers.

Mr. Rivera's experience illustrates why U.S. citizens aren't clamoring for these jobs. On a recent day at noon, the 51-year-old trucker parked his rig in a transportation company's staging yard to wait for an assignment. At 3 p.m., a dispatcher gave him a ticket for the day's first container pickup. Mr. Rivera drove to the Long Beach port so he could be among the first to get his load when the harbor opened for its 6 p.m. night shift.

His first load was a container heading to Kmart, which he dropped at a consolidation yard near the port. By 3 a.m. the next morning, Mr. Rivera had made two additional round trips to retailer warehouses and railway depots in Southern California.

Mr. Rivera bought his truck, which has nearly 900,000 miles on the engine, for $15,000, plus about $3,000 more he paid to service a loan. After accounting for fuel, insurance and truck maintenance, he figures he clears about $2,400 a month. He shares a $600-a-month, one-room apartment with his wife, Carlota, as well as the youngest of their five children and a 28-year-old niece.

The worst part of the job, Mr. Rivera says, is sitting in line for a container -- downtime he's only rarely paid for. "Sometimes I wait four or five hours just to collect one little box," he said, speaking in Spanish. He clears his throat often when he talks. Asked about the cause, he mentions the pollution from the hundreds of trucks idling at one time at the port.

Few Americans want these jobs, says Bob Curry, president of California Cartage Co., a privately held Southern California port-logistics company. The per-trip pay structure of port trucking isn't as attractive as the hourly wages more typical of the local trucking business. Already, Mr. Curry estimates, turnover among the 12,000 drivers who work the Long Beach-Los Angeles port complex is about 60 percent each year. "They can haul dirt for more money than cargo," he says.

A cut in the work force could be devastating, as a nationwide work-stoppage earlier this year demonstrated. As few as 10 percent of all truckers at Los Angeles-Long Beach picked up containers on May 1, when pro-immigration groups called a "Day Without Immigrants." Freeways normally jammed with immigrant-driven rigs were virtually free of trucks.

More than half of all imported apparel and shoes bound for U.S. stores move through Los Angeles or Long Beach ports, says Bruce Berton, officer/director of international business consulting at Stonefield Josephson Inc., a textile and apparel accounting firm. Other ports wouldn't be able to pick up the slack on short notice, he says. Consumers would feel shortages within 30 days. "It would seriously disrupt the economy," says Mr. Berton, who is also on the board of the Export Textile Advisor Commission of the U.S. Department of Trade.

The Transportation Security Administration says it is aware of the ID program's potential impact on port work forces. "We will continue to work with industry, port authorities and workers to address it as we roll out this vital security measure," says Darrin Kayser, manager of strategic initiatives for the TSA.

Demand for port truckers has soared in recent years, as maritime transportation has been revolutionized by containers -- large, stackable steel boxes that are easily transferred between ships, trucks and rail cars. More than 14 million twenty-foot-equivalent containers moved through Los Angeles-Long Beach in 2005, compared with 9.5 million in 2000. The ports began operating a night shift last year and expect container traffic to increase again this year. Overall, the value of international trade in and out of the U.S. climbed to $949 billion in 2004, up 29 percent from 2000.

At the Los Angeles-Long Beach ports, truckers have enormous freedom to roam the terminals. On a recent day, after presenting his driver's license and container number at the gate, Guatemalan immigrant Carlos Ruiz rumbled into a Long Beach terminal, which looks like a massive parking lot on the water's edge. He weaved through row after row of containers stacked four high, driving with relief past a line of truckers waiting for their containers to be plucked from a pile. "If I'd been assigned to a container in that other row I'd be stuck here another three hours," Mr. Ruiz said.

His container sat halfway down a stack at Row D. There, Mr. Ruiz waited a few minutes for a crane operator -- whom he called a "flip-iadora," Spanglish for a woman who "flips" containers -- to lower a 40-foot orange container onto his truck. His rig trembled at the 20-ton impact.

The drivers have no idea what the sealed containers hold. Mr. Ruiz, en route to a Target distribution center in nearby Carson, could have been hauling plastic toys or millions of dollars worth of plasma TVs. "I know what's sold at Target, but I have no idea what's in this container," he said.

As Mr. Ruiz headed toward the gate, the number on his container was read by an optical camera and checked against his delivery ticket. A taped message informed him, in English and Spanish, that he was free to exit the terminal. After delivering his load to Target he began the cycle again, picking up an empty container from the retailer's yard and returning it to the port.

The unescorted terminal access of Mr. Ruiz and other drivers is what worries some security officials. They're concerned that truckers, whose vehicles aren't inspected when they enter a terminal, could be persuaded to carry explosives inside, or facilitate passage of lethal cargo. "If a port is a target, these extremely low-income truck drivers are susceptible to bribery," says Jon Haveman, a research fellow at the Public Policy Institute of California, a nonpartisan institute that studies state social, political and economic issues.

Experts in security say that lapses abound at the seaports. Port truckers aren't allowed to bring unauthorized guests into a marine terminal, for example. But recently one trucker entered Los Angeles port with his adult son sitting beside him. Both father and son, who asked not to be identified, said that the son frequently rides along.

Immigrant advocates and port experts say truck drivers rank far down the list of potential security threats. The big concern is that nuclear devices or other weapons could be smuggled from abroad and detonated on U.S. shores. U.S. Customs and Border Protection, a Homeland Security division, is addressing smuggling: The agency has struck accords with foreign governments to station U.S. inspectors abroad to examine some U.S.-bound containers. The program is operating at 50 ports so far. On Friday, President Bush signed a law that would place radiation-detection equipment at 22 U.S. ports by the end of next year.

Unauthorized workers often carry false Social Security numbers or work under the names of others. But bosses at the area's dozens of trucking companies say they're not in the position to verify whether documents provided by employment-seeking immigrants are authentic. Such undocumented drivers account for as many as half of the port-trucking work force nationwide, estimates Michael H. Belzer, professor of industrial relations at Wayne State University. By comparison, more than 50 percent of crop workers are undocumented, according to the U.S. Labor Department.

Port trucking hasn't always been so unappealing to American workers. Decades ago, when the jobs fell under Teamsters union protection, they offered middle-class wages and benefits. But in the 1970s, major industrial shippers lobbied for an open market to reduce the cost of transporting goods.

When the government deregulated trucking in 1980, nonunion, mom-and-pop trucking firms rushed in. Many union shops folded and most unionized truckers left the ports. Pay and work conditions soon deteriorated, according to Wayne State's Mr. Belzer, who wrote a book on the transformation, "Sweatshops on Wheels." Many of the jobs were taken by Hispanic immigrants.

To protest poor conditions, about 6,000 Latino port truckers in Los Angeles and Long Beach stopped driving in the spring of 1996. Shipping companies and trucking firms were unresponsive and ultimately the campaign failed. "I ruined my credit, lost my car and the trucking company didn't want me back," says Enrique Aguilar, a veteran trucker who stayed away for four months.

A survey conducted in 2004 by Kristin Monaco, a professor at California State University, Long Beach, found that port truckers in Los Angeles-Long Beach earned a median $25,000 a year after expenses. By comparison, unionized longshoremen, who unload containers from incoming vessels, make about $110,000.

Trucking companies say they have little power to raise wages to attract new drivers, because new firms are constantly cropping up and offering lower transport rates to importers. Margins are razor-thin. Averting a longer-term driver shortage would require trucking companies to raise pay enough to make driving jobs more attractive. But that would mean charging higher fees, which the trucking companies say importers and freight forwarders are reluctant to accept.

It's not clear whether consumers would feel the impact of a price hike. Currently, for a typical pair of shoes, freight, duty and brokerage charges come to about $1.15, says Mr. Berton of Stonefield Josephson. Of that, he says, the cost of transporting the shoes from port to distribution center is minimal -- about one cent per pair.