WASHINGTON – Congress approved a major ports security bill early Saturday, providing new steps to prevent terrorists from slipping a nuclear, chemical or biological device into one of the 11 million shipping containers entering the nation every year.

Passage of the bill was the last act of the House as lawmakers left for a five-week election campaign during which candidates will be trying to prove to voters their commitment to keeping America safe in the war on terrorism. The Senate passed it by a voice vote, sending it to the president for his signature.

Containers, now largely uninspected, “have the potential to be the Trojan Horse of the 21st century,” said Sen. Susan Collins, R-Maine, chairman of the Homeland Security and Governmental Affairs Committee. She said the legislation would be a “major leap ahead” in strengthening national security.

Democrats favored the bill, but said it failed to address rail and mass transit, other areas considered highly vulnerable to terrorist attack.

“The terrorist attacks on rail and transit systems in Spain, London and Mumbai [Bombay] should be enough evidence to convince the Republican-led Congress that U.S. rails are dangerously vulnerable,” said Sen. Joe Lieberman, D-Conn.

The bill approves $400 million a year over five years for risk-based grants for training and exercises at ports. It requires the nation’s 22 largest ports, which handle 98 percent of all cargo entering the country, to install radiation detectors by the end of next year.

Pilot programs would be established at three foreign ports to test technology for nonintrusive cargo inspections. Currently only one foreign port, Hong Kong, scans all U.S.-bound cargo for nuclear materials.

Background checks and credentials will be required for workers at the nation’s 361 ports, and the Homeland Security Department would set up protocols for resuming operations after an attack or incident. It is feared that a terrorist attack, such as a nuclear device set off by remote control, could cripple the entire economy as well as cause massive casualties.

Preferential cargo processing is offered importers who meet certain security requirements.

The bill would authorize $3.4 billion over five years for ports security. The House vote was 409-2, with only Reps. Edward Markey, D-Mass., and Jeff Flake, R-Ariz., opposing.

The bill was slow in reaching the House and Senate floors because lawmakers from both sides sought to attach their own favorite pieces of legislation to the ports measure because of the certainty it would reach the president’s desk.

In the end, the only major add-on was legislation to restrict Internet gambling. Also attached was a measure, pushed by Sen. David Vitter, R-La., to help communities lacking telecommunications infrastructure install sirens and other emergency alert systems.

With an eye to the election, Congress has concentrated on security-related issues in the past two weeks, considered measures on military tribunals, President Bush’s wiretapping program, spending for defense and homeland security and a bill to build a fence along the Mexican border.