Sen. John F. Kerry criticized President Bush here on Tuesday for failing to take more aggressive action to reduce the threat of a nuclear attack by terrorists, and the Democrat pledged to lead an effort to secure the world's nuclear bomb-making materials within four years if he is elected president.

The senator from Massachusetts described the threat of nuclear materials falling into the hands of terrorists as the greatest danger facing the United States and said more forceful presidential leadership and a more ambitious timetable for reducing the risk are needed to keep the country safe.

Speaking at the Port of Palm Beach in Riviera Beach, Fla., with a container ship as a backdrop, Kerry invoked images of Cold War dangers to underscore his contention that the nuclear threat has a new face in this era of terrorism.

"The question before us now is what shadowy figures may someday have their fingers on a nuclear button if we don't act," he said. "It is time again that we have leadership at the highest levels that treats this threat with the sense of seriousness, urgency and purpose it demands."

In the face of evidence that terrorist groups, such as Osama bin Laden's al Qaeda network, seek to obtain bomb-making material, Kerry said, the Bush administration has gone backward since the terrorist attacks of Sept. 11, 2001, in securing such materials in the former Soviet Union. He said that at its current pace, the Bush administration's program to secure nuclear materials there would take 13 years to complete. "We simply can't afford another decade of this danger," he said. "My plan will safeguard this bomb-making material in four years."

Bush campaign officials contested Kerry's claims that the administration has not made nuclear nonproliferation a major priority. Speaking for the campaign, Richard Falkenrath, former White House deputy director for homeland security, said Bush has "pushed harder on the nonproliferation agenda than any other president and accomplished more," including an agreement by Libya to dismantle its nuclear program.

He also described Kerry's proposals as "hollow promises" that included "preposterous claims," and he said Kerry had offered no new idea for gaining greater cooperation from a Russian government that does not accept the U.S. view of how serious the problem is.

As Kerry campaigned in Florida, Vice President Cheney was in Kansas City, where he defended the Patriot Act and criticized Kerry for questioning its effectiveness. Cheney said that rather than infringing on civil liberties, the law "has helped us to defend our liberty."

Kerry's speech here was the second of a series he is giving as part of an 11-day focus on defense and national security issues. Last week in Seattle, he outlined the principles of a Kerry administration foreign policy in a speech critical of the president for trying to bully the world rather than establish international alliances. On Thursday in Independence, Mo., the senator will focus on restructuring the military to meet the threats of terrorism.

The speech came as Kerry's campaign unveiled a 30-second television commercial that will air in 19 battleground states. The new ad says that "a stronger America starts at home" and says Kerry has "real plans" for creating jobs and lowering the cost of health care.

Kerry has talked about the dangers of nuclear nonproliferation, but until Tuesday he had not provided a strategy to reduce the threat that nuclear materials pose. He argued that terrorists do not have the capability to produce nuclear bomb-making materials by themselves and that, if the world acts together under U.S. leadership, existing stocks can be keep out of terrorist hands.

The presumptive Democratic presidential nominee said a program to safeguard all nuclear weapons and materials should begin by accelerating efforts to secure nuclear stocks in the former Soviet Union, where nearly 20,000 weapons, and the material to produce another 50,000 nuclear bombs of the size used on Hiroshima in World War II, remain.

Kerry rebuked Bush for failing to take more aggressive action in his meetings with Russian President Vladimir Putin to force the Russians to overcome bureaucratic obstacles and said he would "seek an agreement to sweep aside the key obstacles."

Kerry also said he would attempt to clean out the highly enriched uranium that exists in research reactors in many countries, and said the administration's 10-year timetable should be reduced to four years. He called for strengthening the nuclear Non-Proliferation Treaty and establishing international standards for safeguarding nuclear materials.

Signaling another break with the administration, Kerry said he would halt development of a new generation of nuclear weapons by the United States, including so-called "bunker-busting bombs," saying the United States must lead by example in reducing the nuclear threat.

Citing the dangers that exist in North Korea and Iran, Kerry said the United States must lead a global effort to prevent those nations from developing nuclear weapons.

Kerry said he would keep all options on the table for dealing with North Korea, but he reiterated his criticism of the Bush administration for its unwillingness to engage in direct talks with the North Koreans, saying he would be open to such discussions in addition to the six-party negotiations underway. On Iran, he said the administration's preoccupation with Iraq has hindered efforts to curb that nation's nuclear program.

Finally, Kerry said he would create a White House-level position to coordinate the nuclear terrorism prevention effort, to elevate the significance of the program in his administration.

Kerry offered no cost estimates for his plan. Graham Allison, a professor at Harvard's Kennedy School of Government, who accompanied Kerry to Florida, said the United States spends about $1 billion a year on efforts to safeguard nuclear materials and that, under Kerry's accelerated timetable, the cost could rise to between $5 billion and $6 billion annually. He said the United States could get other nations to help share the cost.

Sen. John F. Kerry (D-Mass.) discussed his national security plan Tuesday at Florida's Port of Palm Beach.