WASHINGTON – The Senate voted unanimously on Thursday to tighten security at U.S. seaports by scanning nearly all incoming cargo for nuclear weapons or “dirty bombs.”

The bill, approved 98-0 in a pre-election push on national defense, would increase safeguards on the rail systems that pick up cargo from ports and authorize 1,000 new agents to screen containers coming off ships.

“The link between maritime security and our national security is obvious, and the vulnerabilities of our ports is worrisome,” U.S. Sen. Susan Collins, R-Maine, one of the bill’s authors, said after its passage.

“This important legislation will help close dangerous gaps in our ability to protect our shipping lanes and seaports from attack,” she said.

But the legislation does not go as far as some Democrats demanded in requiring inspections for all U.S.-bound cargo before it leaves foreign ports. Almost 11 million containers are shipped annually to the United States.

The plan, which authorizes spending $835 million next year, “works toward a goal of getting to 100 percent screening” of cargo leaving foreign ports, said Sen. Patty Murray, D-Wash., the bill’s other author.

Sen. Ted Stevens, R-Alaska, said the measure, was “the most comprehensive approach to border security we have taken to date.”

The bill includes elements not covered in a House plan passed in May. Still, Collins, the head of the Senate Homeland Security and Governmental Affairs Committee, said she is optimistic that a compromise plan will pass before the end of the year.

U.S. Rep. Michael Michaud, D-Maine, said he would continue to press his colleague in both houses to reach an agreement.

“We must get our homeland security personnel the tools they need to keep our country safe,” Michaud said in a statement after the vote.

President Bush said in a statement the Senate bill would “strengthen my administration’s efforts to secure our ports and detect dangers before they reach America’s shores.”

The Senate bill requires inspections of suspicious high-risk cargo at foreign ports. It also sets up a pilot program to scan for nuclear or “dirty bomb” materials in all U.S.-bound containers at three to-be-determined foreign ports. The trial would help determine whether mandatory inspections would bottle up commerce and drive up costs, as Republicans fear.

Sen. Charles Schumer, D-N.Y., said that could be too late to prevent massive explosions at U.S. ports or in harbors outside large cities.

“We don’t want to be in a situation where we say, ‘What if? What if we had done more?”‘ Schumer said.

Responded Sen. Norm Coleman, R-Minn.: “This about the practicality of making sure that we have something that works.”

Noting the politics of the issue, Coleman added: “The election season is upon us. It’s getting very close. … Let us step away from the sloganeering.”

The president’s Homeland Security chief has said that nuclear weapons are the gravest threat facing the country, and experts fear they could be smuggled easily into the U.S. in shipping cargo.

Such weapons create huge fireballs fed by nuclear chain reactions. “Dirty bombs,” in contrast, would use conventional explosives to scatter radioactive material. This kind of blast probably would not cause many deaths, experts say, but the fear of contamination could spark panic while land and buildings hit with radioactive particles might be unusable for years.

James Jay Carafano, homeland security fellow at the Heritage Foundation, said the Senate plan largely ignores security gaps such as the threat from small boats that could detonate explosives next to larger ships, as happened in the 2000 attack on the USS Cole in Yemen that killed 17 Americans.

The bill would make available $400 million for security grants and expedite incoming cargo from previously approved manufacturers and other business partners. It also included special items for landlocked states, including $40 million for northern border air patrols over Great Falls, Mont., requested by Sen. Max Baucus, D-Mont., and extending the daily hours at a New Mexico border crossing, sought by Sen. Jeff Bingaman, D-N.M.

The administration has spent about $10 billion on ports security since the attacks of Sept. 11, 2001. About 65 percent of cargo is already screened for nuclear or radiological materials. The Homeland Security Department aims to increase that number to 80 percent by the end of the year and to almost 100 percent by the end of 2007.