US Vice-President Dick Cheney has said a vote for Democratic presidential candidate John Kerry could make a terror attack on the US more likely.
Cheney said the Republicans had made the right choices in Iraq
"If we make the wrong choice, then the danger is that we'll get hit again," Mr Cheney told in a rally in Iowa.
Mr Kerry's running mate John Edwards accused the Republicans of scare tactics that "crossed the line".
Both parties have been stepping up their campaigning ahead of presidential elections on 2 November.
Mr Cheney, speaking to Republican supporters in Des Moines, said his party was better equipped to fight terrorism.
If Mr Kerry were elected, he said, the country risked falling back into a "pre-9/11 mindset", believing that terror attacks were just criminals acts "and that we're not really at war".
Mr Cheney also defended the White House's record in Iraq, saying it had removed from power a leader who used weapons of mass destruction against his own people and harboured terrorists.
"Saddam Hussein today is in jail, which is exactly where he belongs," he added.
Mr Edwards hit back, saying Mr Cheney's comments showed "he and George Bush will do anything and say anything to save their jobs".
"Protecting America from vicious terrorists is not a Democratic or Republican issue, it's an American issue, and Dick Cheney and George Bush should know that," he added.
Campaigning has intensified after last week's Republican convention, when opinion polls showed Mr Kerry slipping behind President George Bush in the race for the White House.
Mr Kerry's aides have called it a short-lived bounce that would dissipate over the next few weeks.
Kerry has accused Bush of rushing into war
However, Mr Kerry has upped the tempo in recent days of his criticism of Mr Bush's handling of the war in Iraq.
Campaigning in North Carolina, Mr Kerry said the president had made a "catastrophic" mess of the war in Iraq, and that the resulting costs were depleting the national budget.
"The price tag so far - $200bn and rising. That's $200bn that we're not investing in health care in America, that's $200bn we're not investing in schools in America, that's $200bn that were not
investing in prescription drugs for seniors," the Massachusetts senator told Democratic supporters in Greensboro.
On a bus trip across Missouri, Mr Bush also stepped up his criticism of Mr Kerry for what he said were shifting positions on the war in Iraq.
"No matter how many times Senator Kerry flip-flops, we were right to make America safer by removing Saddam Hussein from power," he said.