Polls showed the race a virtual dead heat with Bush maintaining a statistically insignificant lead on a national level. The battle for decisive electoral votes awarded on the state level was still too close to call.

With only seven days left before the crucial vote, both candidates on Tuesday launched into a marathon of rallies with Bush traveling to Wisconsin and Iowa and Kerry visiting Wisconsin, Nevada, New Mexico and Iowa.

Bush on Monday avoided speaking about the hundreds of tons of explosives missing in Iraq, but castigated Kerry for criticizing the US-led invasion.

Kerry "calls America's missions in Iraq a mistake, a diversion, a colossal error. Then he says he's the right man to win the war," said Bush. "You cannot win a war you do not believe in fighting."

"On Iraq, my opponent has a strategy of pessimism and retreat," the president said. "He has sent the signal that America's overriding goal in Iraq would be to leave, even if the job is not done."

But Kerry pounced on reports by the International Atomic Energy Agency (IAEA) that 350 metric tonnes (380 tonnes) of explosives had vanished from an unguarded military site shortly after the US-led invasion in March 2003.

The UN's nuclear watchdog said the material could be used in conventional bombs or to trigger an explosion in a nuclear device.

"George W. Bush, who talks tough, talks tough, and brags about making America safer has once again failed to deliver," Kerry said in a rally in Dover, New Hampshire.

"After being warned about the danger of major stockpiles of explosives in Iraq this president failed to guard those stockpiles.

"Terrorists could use this material to kill our troops, our people, blow up airplanes and level buildings," Kerry said.

"This is one of the great blunders of Iraq, one of the great blunders of this administration, Kerry said, adding that "incredible incompetence" by Bush had put US troops and the United States at great risk.

Bush aides scrambled to contain the damage, saying the White House had only learned of the missing explosives in early October, and charging that Kerry lacked a clear vision for prosecuting the war on terror.

A Pentagon spokesman said it was unclear whether the missing explosives disappeared before or after Iraq fell under control of US forces.

"This is a first report. We do not know when -- if those weapons did exist at that facility -- they were last seen, and under whose control they were last in," Pentagon spokesman Larry DiRita said.

"It's very possible -- certainly it's plausible -- that it was the Saddam Hussein regime that last had control of these things," he told AFP.

Critics have long said too few troops were deployed to secure Iraq after the collapse of Saddam Hussein's regime. They blame poor planning and the mistaken assumption that Iraqis would welcome US-led forces with open arms.

A new Gallup poll for CNN/USA Today showed Bush may have a slight lead but that the election is still too close to call -- 51 percent of likely voters back Bush and 46 percent Kerry. But with a margin of error of plus or minus three percentage points, the race leader was "unclear".

Meanwhile, Clinton -- looking thinner but smiling broadly -- made his first public appearance Monday since undergoing quadruple bypass heart surgery seven weeks ago, to add star power to the sometimes monotone Kerry campaign.

"From time to time I have been called the comeback kid," the politically savvy former president told a rally of cheering supporters. "In eight days John Kerry is going to make America the comeback country."

The Democrats hope to use Clinton's enormous popularity among minorities and other constituencies, as well as his eight-year record of economic prosperity and fiscal prudence, to solidify Kerry's base and reach out to swing voters.

Clinton ticked off a litany of alleged Bush administration failings on jobs, taxes, health care and foreign policy, and sought draw to draw a sharp contrast between Kerry and the Republicans.

"Our friends on the other side want a world where they concentrate wealth and power on the far right, do what they want to when they can and cooperate with others only when they have to," he said.

Clinton, who later flew to Miami for a get-out-the-vote rally, told ABC television he was throwing himself into the campaign because it was so close.

"I think this is one of the most difficult elections to call I have ever seen," said Clinton.

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