Nationwide, polls show the race continues to be close, with Bush enjoying a narrow lead. In the latest FOX News/Opinion Dynamics poll, Bush is ahead five points in a two-way race — up from three points earlier this month.
Bush said in Mason City, Iowa, on Wednesday that Kerry's views on national security are so misguided that the Democrat would be unable to defeat terrorism.
"The next commander in chief must lead us to victory in this war and you cannot win a war when you don't believe you're fighting one," Bush told hundreds of supporters in a northern Iowa farming community. "My opponent also misunderstands our battle against insurgents and terrorists in Iraq, calling Iraq 'a diversion from the War on Terror.'"
Bush criticized a recent comment by Kerry that the events of Sept. 11 hadn't changed him much and a comment by Kerry's top foreign policy adviser that the nation is not in a War on Terror in a literal sense.
The Kerry campaign is guilty of "a fundamental misunderstanding of the war we face and that is very dangerous thinking," the president said.
Bush said the case of terrorist Abu Musab al-Zarqawi (search), who is engaging in beheadings of Americans in Iraq and has pledged his allegiance to Usama bin Laden, "shows how wrong" Kerry's thinking is.
"If Zarqawi and his associates were not busy fighting American forces, does Sen. Kerry think he would be leading a productive and useful life?" asked Bush. "Of course not. And that is why Iraq is no diversion."
During his speech in Waterloo, Iowa, Wednesday, Kerry focused on Iraq.
"I will lead and I believe others will follow," the Massachusetts senator said. "The president says he's a leader. Well, Mr. President, look behind you. There's hardly anyone there. It's not leadership if we haven't built the strongest alliance possible and if America is going almost alone."
Kerry's speech comes on the heels of blistering statements from the president early this week in which the Republican incumbent said the senator stands for "protest and defeatism" in Iraq and that Kerry would lead the nation toward "a major defeat in the War on Terror."
Kerry said the statements show an administration running from its record.
"He wants to make it solely a contest on national security. Well, I welcome that debate," Kerry said. "I believe a president must be able to defend this country and fight for the middle class at the same time."
Kerry said U.S. leaders must stop treating other countries with "contempt," driving them away from a role in Iraqi security and reconstruction. He also repeated charges that the president's conduct made the U.S. weaker, not stronger, in the War on Terrorism.
"America is fighting and must win two wars. The war in Iraq, and the War on Terror," he said. "Iraq was a profound diversion from that war and the battle against our greatest enemy, Usama bin Laden and the Al Qaeda network. But now that we're fighting two wars, we must and we will prevail in both."
After Iowa, Kerry held a rally in Pittsburgh with supporters, including singer Jon Bon Jovi, actor Ted Danson and Hall of Fame football player Franco Harris, before retiring to Ohio for the night. Four years ago, Bush lost Pennsylvania and won Ohio; this year the race is tight in both states.
Kerry aide Mike McCurry said Kerry's call for new leadership in Iraq is one prong of their final arguments to sway undecided voters who see Bush on the wrong track.
"They are more and more convinced that President Bush does not deserve to be re-elected, and they are trying to get to the point where they can see and embrace John Kerry as the next president," he said.
White House spokesman Scott McClellan said of Kerry's speech: "Last week they said they were going to focus on domestic issues for the remainder of the campaign ... There is a very clear choice in how to lead in the War on Terror."
Kerry "has a pre-9/11 mindset" that exposes a "fundamental misunderstanding of the War on Terror," McClellan continued. "[Kerry's] is an approach based on responding to attacks. The president has a strategy to prevent attacks."
Over the last several days, the vaccine shortage was injected squarely into the presidential race, as Bush defends his administration and Kerry tries to hold him responsible for the loss of nearly 50 million doses of vaccine — half this season's expected supply.
Kerry, the Democratic presidential nominee, released a radio advertisement Tuesday stoking fears over the shortage as congressional Democrats blamed Bush for not addressing well-documented problems in the vaccine industry.
Bush responded in Florida, telling voters he knows they are worried. And administration health officials trumpeted news Tuesday that an additional 2.6 million doses of vaccine will soon be available.
"If you're an elderly man or woman, if you're a young child, if you're a pregnant woman, George Bush and the Republicans have this to say on health care: Don't get sick," says Kerry's ad, which followed a similar spot for TV that has yet to air.
Bush tried to reassure: "We have millions of vaccine doses on hand for the most vulnerable Americans, and millions more will be shipped in the coming weeks," he said in St. Petersburg, Fla., a state heavy with senior citizens vulnerable to the sometimes deadly virus.
Each candidate worked to integrate his message into the broader themes of his campaign. For Kerry, this is another example of Bush's incompetence: first the war in Iraq, now flu shots at home. For Bush, the blame goes to runaway verdicts. He argues that more manufacturers would be in the vaccine business if they didn't have to worry about lawsuits from people who have adverse reactions to inoculations.
The Associated Press contributed to this report.