President Bush and John F. Kerry unleashed sharp new attacks over national security on Monday, as Kerry called the president incompetent for failing to safeguard deadly explosives in Iraq and Bush accused his challenger of lacking confidence and resolve in moments of crisis.

Seizing on new reports of mass killings and missing stockpiles of highly dangerous weapons in Iraq, Kerry interrupted a day dedicated to former president Bill Clinton's ceremonial return to the political stage to blame Bush's leadership for the disappearance of nearly 380 tons of explosives in Iraq.

"George W. Bush talks tough and brags about making America safer, but once again he has failed to deliver," Kerry told tens of thousands of supporters crammed into the streets of Philadelphia to hear him and the former president. It was the largest U.S. crowd ever to greet Clinton. "Today America has learned that those stockpiles are missing, are unaccounted for and could be in the hands of terrorists [who] can use this material to blow up our airplanes, blow up our buildings, kill American troops."

Bush, campaigning in Greeley, Colo., did not address the disappearance, which the International Atomic Energy Agency confirmed Monday, and said the Pentagon reported that the missing explosives are only a fraction of the known weapons stockpiles in Iraq.

The president portrayed Kerry as out of step with his own party on security matters. "He stands in opposition not just to me, but to the great tradition of the Democratic Party," Bush said in an appeal to Democrats.

"The party of Franklin Roosevelt and Harry Truman and John Kennedy is rightly remembered for confidence and resolve in times of war and in hours of crisis. Senator Kerry has turned his back on 'pay any price' and 'bear any burden.' And he has replaced those commitments with 'wait and see,' and 'cut and run.' "

The grim news from Iraq -- that 49 Iraqi National Guard recruits had been slaughtered and that U.S. authorities had not secured the explosives -- came at a particularly unwelcome time for the Bush campaign. The president was barnstorming with former New York mayor Rudolph W. Giuliani and delivering what the president's staff had billed as a major new speech about terrorism, laced with harsh attacks on Kerry.

Instead, the news about the missing explosives, first reported by the New York Times and CBS News, gave Kerry an opening on national security -- Bush's strength -- as the candidates began their final week of campaigning, eight days before the election. Starting with an e-mail sent to reporters before 1:30 a.m., Kerry and his supporters pounced on what Kerry called "one of the greatest blunders of this administration."

It was the second day in a row that Bush appeared on the defensive on the issue his advisers believe will lead to Kerry's defeat: keeping the United States safe. On Sunday, Democrats criticized Bush for saying it is "up in the air" whether the United States would ever be safe from terrorists.

White House press secretary Scott McClellan said the president "wants to make sure that we get to the bottom of this," but also said Kerry was exaggerating the danger of the missing weapons and the administration's culpability. "The first priority, for our standpoint, was to make sure that this wasn't a nuclear proliferation risk, which it is not," McClellan said. "These are conventional high explosives that we are talking about."

The Pentagon reported that the explosives in question -- mainly bomb-making substances called HMX and RDX -- had been used in past terrorist attacks overseas. To date, the Pentagon said it has destroyed 243,000 tons of weapons and secured 163,000 tons that are awaiting destruction. White House communications director Dan Bartlett said the administration was aware of the missing weapons but did not make the news public because it wanted "to get all the facts and find out exactly what happened" first. Democrats suggested a cover-up.

The Bush campaign pushed reporters to look into an NBC News report that the network had been embedded with troops who searched the site three weeks into the war but never found the powerful explosives that are now missing, suggesting they were already gone.

With polls showing a virtual tie nationally and in key states, both sides are bracing for the unknown -- developments in Iraq, the potential capture or catastrophic acts of terrorists, or damaging revelations about one of the candidates -- that could move enough votes to make a difference. For now, Bush and Kerry are talking about a more positive finish, though there were few signs of that Monday.

Parrying Kerry's accusation that Bush supported "the wrong war in the wrong place at the wrong time," the president said his "opponent has the wrong strategy for the wrong country at the wrong time."

Later, in Council Bluffs, Iowa, Bush rebutted Kerry's criticism that he had left too much of the hunt for Osama bin Laden to Afghan warlords, calling that "an unjustified criticism of our military commanders in the field" and "the worst kind of Monday morning quarterbacking, and that's what we've come to expect from Senator Kerry."

Sen. John Edwards (N.C.), Kerry's running mate, speaking to supporters in Toledo, accused Bush of "grave incompetence" in Iraq and said the president's failure to secure weapons in Iraq could be catastrophic. "About one pound of this stuff was what it took to bring to down Pam Am Flight 103," he told a union crowd of close to 3,000. "These explosives could be used to take down more buildings and airplanes, to harm our troops and to detonate a nuclear weapon."

Edwards also went after national security adviser Condoleezza Rice for speaking in the battleground state of Florida on Monday instead of "figuring out what went wrong and . . . how we're going to find this material."

Vice President Cheney, in Minnesota, told voters that a "breaking point in American history" has been reached, in which leaders must set a course for U.S. security for the next three to four decades.

Even Clinton joined the debate over terrorism and fear. Making his first public appearance since having heart-bypass surgery, a thinner and more laconic Clinton implored the crowd here to choose hope over the fear he said Bush and Cheney are pushing. "The other side, they're trying to scare the undecided voters about Senator Kerry," he said. "And they're trying to scare the decided voters away from the polls."

Kerry, who stumped in New Hampshire, Pennsylvania, rural Michigan and Green Bay, Wis., on Monday, was not as polished as Clinton, but he was more pointed. The senator from Massachusetts said in Philadelphia that Bush "failed the test of commander in chief" by not having a better plan for a postwar Iraq and by not averting mounting casualties and chaos there.

He went on to slam the president as a one-trick politician who is trying to scare Americans.

Bartlett said Kerry's attacks, especially over Iraq, are not hurting the president's standing with voters. "Regardless of how Senator Kerry tried to attack President Bush, the public has confidence in President Bush's approach to it. If he wants to debate Iraq and the war on terror every day up to the election, we're more than happy to do so," he said.

Allen is traveling with Bush. Staff writers Lindsey Layton, with Cheney, and John Wagner, with Edwards, contributed to this report.

In his first public appearance since having heart bypass surgery, former president Bill Clinton joined Sen. John F. Kerry on the campaign trail in Philadelphia.President Bush portrays Kerry as out of step with his own party on security.