The disappearance of nearly 400 tons of explosives in Iraq dominated the presidential race for a third straight day on Wednesday, as Democratic nominee John F. Kerry accused President Bush of evading responsibility and the Republican said Kerry was making unsubstantiated charges.

Kerry, traveling in Iowa, scrapped plans to talk about domestic policy to accuse Bush of trying to cover up the failure to secure the explosives in Iraq. "This is a growing scandal and the American people deserve a full and honest explanation of how it happened and what the president is going to do about it," Kerry told supporters in Sioux City. Instead, he said, "we're seeing this White House dodging and bobbing and weaving . . . just as they've done each step of the way in our involvement in Iraq."

Bush, breaking two days of silence on the issue, told supporters at a rally here that Kerry was making "wild charges" about the missing munitions and was "denigrating the action of our troops and commanders in the field without knowing the facts."

"Our military is now investigating a number of possible scenarios, including that the explosives may have been moved before our troops even arrived at the site," Bush said, adding: "A political candidate who jumps to conclusions without knowing the facts is not a person you want as your commander in chief."

That Bush addressed the issue at all -- on Tuesday he only glared at a reporter who inquired about the matter -- reflected the prominence the explosives have gained in the final days of the presidential race, when every moment is precious to the campaigns. Kerry has used the situation to question Bush's terrorism-fighting credentials, and the matter has crowded out the subjects Bush is raising, particularly an appeal to Democrats.

The candidates' dispute centers on when the munitions disappeared from an Iraqi weapons depot -- before or after Saddam Hussein's fall from power last year. Details about which U.S. troops were among the first to reach the depot and what they did there have dribbled out of the Pentagon.

On Wednesday, the Pentagon said members of the 3rd Infantry and 101st Airborne divisions stopped there in early April 2003 but pushed on to Baghdad without hunting for the explosives. The first designated search teams, members of the 75th Exploitation Task Force, surveyed the depot on May 8, 11 and 27 in 2003 but found none of the explosives in question, the Pentagon said.

While there has yet to be an "October surprise" that shakes up the race, a series of small, negative surprises have undermined Bush's campaign: the flu-vaccine shortage, climbing oil prices, falling stocks and this month's disappointing jobs report.

On Tuesday, Iraqi interim Prime Minister Ayad Allawi accused the U.S.-led forces in Iraq of "gross negligence" in allowing the massacre of 49 Iraqi guardsmen by insurgents. That followed the news Monday that the International Atomic Energy Agency had told the Bush administration on Oct. 15 that about 380 tons of powerful explosives had vanished from the depot.

Recently, Kerry has jumped from bad headline to bad headline to calibrate his attacks on Bush. For the most part, he is making this race a mandate on Bush measured by public reports and statistics, relegating his own ideas largely to the closing moments of 40-minute speeches. Kerry aides privately acknowledge that the Democrat has not captured the imagination of many voters. But they argue that he has successfully presented himself as an acceptable alternative to Bush.

While the bad news does not necessarily translate into support for the challenger, a survey of swing voters released Wednesday by the nonpartisan Pew Research Center found that Kerry "has made more substantial gains among these swing voters in the past month." Pew reported that the number of swing voters leaning toward Kerry increased to 40 percent from 28 percent, while the number supporting Bush increased to 38 percent from 34 percent.

To quiet audiences on Wednesday, Bush sought to use the missing munitions to his advantage by suggesting their existence, though conventional, confirmed the case for war. "After repeatedly calling Iraq the 'wrong war' and a 'diversion,' Senator Kerry this week seemed shocked to learn that Iraq was a dangerous place full of dangerous weapons," he told supporters. "The senator used to know that, even though he seems to have forgotten it over the course of the campaign. But after all, that's why we're there."

Bush said U.S.-led forces have destroyed or captured more than 400,000 tons of munitions, and he pointed to remarks by Kerry foreign policy adviser Richard C. Holbrooke. "Now the senator is making wild charges about missing explosives when his top foreign policy adviser admits, quote, 'We do not know the facts,' " he said. "Think about that. The senator's denigrating the action of our troops and commanders in the field without knowing the facts."

Aides said Bush was referring to an interview Holbrooke gave Fox News on Tuesday in which he said: "The U.N. inspectors and the IAEA inspectors had told the American military this was a major depot. . . . Now the thing has been looted. I don't know what happened. I do know one thing: In most administrations, the buck stops in the Oval Office."

Bush's remarks produced a furious response from Democrats. In a conference call arranged by the Kerry campaign, Sen. Joseph R. Biden Jr. (Del.) issued an extraordinary rebuke of the administration. Because of missing explosives, "my kid and a lot of other kids might get their ass over there and get blown up by these because of their civilian incompetence," he said, his voice rising to a shout.

Campaigning in Florida, Kerry's running mate, Sen. John Edwards (N.C.), kept the emphasis on the missing explosives. "What is he talking about?" he said of Bush's charge that Kerry is "denigrating" the troops. "Aren't we sick and tired of George Bush and Dick Cheney using our troops as shields to protect their own jobs instead of doing everything they should to protect our troops? Our men and women in uniform did their job. George Bush didn't do his."

Vice President Cheney, also in Florida, kept his focus on security, too, charging that Kerry "is trying every which way to cover up his record on defense, which is one of weakness." Standing near a 1942 Stearman biplane that George H.W. Bush flew as a young man, Cheney said in Kissimmee: "President Bush understands the war on terror and has a strategy for winning it; Senator Kerry does not."

The Kerry campaign said the larger issue is Bush's failure to secure postwar Iraq. "He's doomed to make the same mistakes all over again," Kerry said. "Three hundred and eighty tons of explosives that could be in the hands of terrorists, and he would do exactly the same?" He called Cheney the "chief minister of disinformation" for leading a White House effort to discredit its critics and blame others for allowing the cache to disappear.

All this week, Bush has been seeking to appeal to Democrats. He appeared Wednesday with Sen. Zell Miller (D-Ga.), who said at a rally near Youngstown, Ohio, that he sees the "ranks growing every day" of Democrats for Bush. Bush said he, not Kerry, follows the strong-defense tradition of Democrats such as Franklin D. Roosevelt, Harry S. Truman and John F. Kennedy. Bush, who met in Michigan on Wednesday evening with more than two dozen African Americans, including boxing promoter Don King, also told supporters he had common views with Lyndon B. Johnson and Hubert H. Humphrey.

In response to Bush's mention of Democrats, the Democratic National Committee issued a statement from Caroline Kennedy Schlossberg, the late president's daughter, saying: "All of us who revere the strength and resolve of President Kennedy will be supporting John Kerry on Election Day."

The tenor of Wednesday and recent days suggests that whatever the original intentions of the campaigns, both sides seem content to let the debate in the final days be about security. In a television ad released Wednesday, the Kerry campaign thanks U.S. forces in Iraq. "As we see the deepening crisis and chaos in Iraq, as we choose a new commander in chief and a fresh start, we will always support and honor those who serve," the ad says.

Bush released an ad with a similar theme, in which he says he will defend the nation, "whatever it takes," and recalls: "I've met with the parents and wives and husbands who have received a folded flag. And in those military families, I have seen the character of a great nation."

While Bush continued to predict victory, he also expressed a note of stoicism. "On good days and on bad days, whether the polls are up or the polls are down, I am determined to win this war on terror and to protect the American people, and I will always support the men and women who wear the nation's uniform," he said.

VandeHei is traveling with Kerry. Staff writers John Wagner, traveling with Edwards; Lyndsey Layton, traveling with Cheney; and Bradley Graham in Washington contributed to this report.

President Bush, who attended a rally in Findlay, Ohio, with wife Laura, broke his silence regarding the missing explosives in Iraq at another rally, accusing his challenger of making "wild charges" without "knowing the facts."Sen. John F. Kerry (Mass.) attends a rally at the Mayo Civic Center in Rochester, Minn. The Democrat shifted plans to talk about domestic policy to accuse Bush of trying to cover up the failure to secure the explosives.Bush's team includes Deputy Chief of Staff Joseph W. Hagin, adviser Karl Rove and deputy national security adviser Stephen J. Hadley.