All that makes ports among the country's greatest points of vulnerability.

Even so, no national plan exists to thwart attacks against them, to respond if one happens or to organize a community afterward. No federal agency regulates seaports the way the Federal Aviation Administration manages airports. They are managed locally, often by the private businesses that use them. All are overseen by a patchwork of agencies, already stretched thin, some monitoring hundreds of ships a day.

Compared with the attention being given to airline security, security at the ports has gone largely unnoticed, even though they handle 95 percent of the cargo that enters from places other than Canada and Mexico. A bill to tighten port security has passed a Senate committee. The full Senate could vote on the bill within two weeks, but the debate has yet to begin in the House of Representatives.

"People in Congress don't have any idea it's a problem," said Senator Ernest F. Hollings, Democrat of South Carolina, who is chairman of the Commerce Committee and co- sponsor of the bill with Senator Bob Graham, Democrat of Florida. "I've got folks who don't have ports in their states. It's hard to get it in front of their heads."

Port officials are aware of various threats, like using a tanker or fuel- loaded cruise liner as a bomb, secreting weapons and explosives in containers, hijacking a ship and ramming it into a nuclear plant on the shores of a river, or infesting a cargo of grain or seeds with a biological weapon.

Given the potential dangers, the security measures in place are far from adequate.

"We're looking for needles in a haystack," said Dean Boyd, a spokesman for the United States Customs Service. "And the haystack has doubled." International trade has doubled since 1995 while the number of people to handle inspections has remained roughly constant, he said.

The Coast Guard patrols coasts and harbors but little of the land or the cargo. It checks out ships coming in from the open sea but has no way of thoroughly searching everything that comes by.

The Customs Service says it can inspect only 2 percent of the 600,000 cargo containers that enter seaports each a day on more than 500 ships. Of the 2 percent, many are not inspected until they reach their final destination, sometimes on the opposite coast, where they travel unguarded by rail, barge and truck.

Last year, a government commission on crime and security at seaports found similar weaknesses. The commission surveyed 12 major ports including those of New York and New Jersey, Miami, Los Angeles, New Orleans and Charleston.

While withholding their identities for security reasons, the report found that only three of the ports tightly controlled access from the land and that access from the water was completely unprotected at nine of them.

The report also emphasized the hazards posed by materials unloaded from ships. "The influx of goods through U.S. ports provides a venue for the introduction of a host of transnational threats into the nation's infrastructures," the report said.

A tangled chain of authority further compromised security, the commission said, a point echoed by the authorities in Portland.

"No one's in charge," said Jeffrey W. Monroe, director of transportation for the city. "There's no central guidance."

And ports have a strong economic incentive to limit control. With the taxes that cruise ships, tankers and other businesses pay, ports are the lifeblood of their communities. Port authorities' principal constituencies are private industry and economic development offices, whose mission is growth, not security. "They win if they move more cargo," Senator Hollings said.

In Portland, the seaport has been a boon, generating millions of dollars a year in revenues. Mr. Monroe said that in the past year the bulk cargo business grew 10 percent, passenger traffic and oil imports both rose by 20 percent. But the stalling economy and now the cost of heightened security have wiped out nearly all that the seaport and airport contribute to the city budget.

In Congress, the Hollings-Graham legislation would help cities meet some of the cost of securing their ports. It would give the Coast Guard regulatory control over ports, require background checks of waterfront workers and provide for 1,500 new Customs agents.

Before the September attacks, the seaport industry's principal lobby, the American Association of Port Authorities, fought the legislation, arguing that it would impose one- size-fits-all security systems for all seaports.

Though the group now supports many provisions of the bill, it still has questions over the matter of who controls security. Meanwhile, ports have taken their own steps to improve security. In Florida, Gov. Jeb Bush announced he would deploy the National Guard to oversee four of the state's busiest ports. In California, Gov. Gray Davis tightened security around bridges.

In Portland, officials and businesses have taken similar steps. Minutes before the drawbridge opens for a tanker, police officers arrive to monitor both sides of the bridge. Fences are being repaired and installed.

At the city's International Marine Terminal, where from May to October the Scotia Prince carries 170,000 passengers on 11-hour cruises between Portland and Yarmouth, Nova Scotia, visitors used to roam freely around the pier. Now only passengers are allowed there, and then only after they and their baggage are cleared by metal detectors and bomb dogs. The pilings below the pier are now illuminated at night.

For its part, the Coast Guard now focuses primarily on harbor security. It requires vessels weighing more than 300 tons to notify the port 96 hours before arrival. The big ships also must fax crew lists, said Lt. Cmdr. Wyman W. Briggs, executive officer of the guard's facilities in Portland. The crews of fishing boats must carry picture ID's.