The US Vice President, Mr Dick Cheney, has begun invoking the spectre of a nuclear attack on American cities to question whether Senator John Kerry is a strong enough leader to cope with such an "ultimate threat" writes Conor O'Clery in Pittsburgh.
"The biggest threat we face now as a nation is the possibility of terrorists ending up in the middle of one of our cities with deadlier weapons than have ever before been used against us - biological agents or a nuclear weapon, or a chemical weapon of some kind to be able to threaten the lives of hundreds of thousands of Americans," he said.
"That's the ultimate threat. For us to have a strategy that's capable of defeating that threat, you've got to get your mind around that concept," he told a rally in Ohio.
The stepped-up attack on Senator Kerry comes as a new Wall Street Journal/NBC News poll shows the presidential race deadlocked at 48 per cent each for President George Bush and Senator Kerry among likely voters. The same poll a month ago gave Mr Bush a 50-46 lead.
Given the ferocity of the attacks on his national security credentials the Democratic challenger has had to adjust his planned speeches on the economy to respond. Mr Kerry repeated on Tuesday evening his scathing criticism of President Bush's invasion of Iraq, calling it a diversion in the war on terror.
At a rally in Wilkes-Barre in Pennsylvania he accused the President of failing to focus on capturing Osama bin Laden, who ordered the 9/11 attack on the US, and said America needed a fresh start with new credibility in Iraq.
"This President likes to say he's a leader," Mr Kerry said yesterday as he campaigned in the battleground states of Florida, Ohio and Pennsylvania on the same day, ending up last night in Pittsburgh. "Mr President, look behind you. There's no one there. It's not leadership if no one follows." US leaders must stop treating other countries with "contempt", he said, driving them away from a role in Iraqi security and reconstruction.
Mr Kerry's frequent assertion that the Bush administration blew a chance to capture bin Laden at Tora Bora has been challenged by Gen Tommy Franks, the officer in control of the US operation in Afghanistan in 2001. He said there was no guarantee bin Laden was there at the time.
Mr Cheney, speaking to ticket-only audiences on a bus tour to energise the Republican base in Ohio, said Mr Kerry was trying to convince voters he would be the same type of "tough, aggressive" leader as the President.
"I don't believe it, I don't think there's any evidence to support the proposition that he would, in fact, do it," said Mr Cheney.
Democrats reacted angrily to the Vice President's raising of the nuclear scare to influence votes. A spokeswoman said: "He has the audacity to question whether a decorated combat veteran who has bled on the battlefield is tough and aggressive enough to keep America safe. "He wants to scare Americans about a possible nuclear 9/11 while the Bush administration has been on the sidelines as the nuclear threats from North Korea and Iran - the word's leading sponsor of terrorism - have increased."
Mr Cheney last month told supporters it was essential to "make the right choice" in the election otherwise "the danger is that we'll get hit again, that we'll be hit in a way that will be devastating".
In a new Kerry advert, a woman whose husband was killed on 9/11 says she voted for Mr Bush in 2000 but will now vote for Mr Kerry because America was no safer. "I want to look in my daughter's eyes and know that she is safe."
Mr Bush, meanwhile, campaigned in Florida where the lack of influenza vaccine has become a big issue, with its large population of elderly people. The US lost half of its supply of vaccines for this winter when the manufacturing plant in England, owned by the Chiron Corporation, had to close because of contaminated vaccines. US Health Secretary Mr Tommy Thompson has admitted that even with replacements, only about 60 per cent of what is needed would be available.
"If you can't get flu vaccines to Americans, how are you going to protect them against bio-terrorism?" asked Senator Kerry. His campaign rushed out an advert saying: "They relied on foreign workers to make the vaccine. A company with a bad track record. Now we're about 50 million vaccines short."
Meanwhile, Mr Kerry's wife, Mrs Teresa Heinz Kerry, has again attracted controversy after she questioned whether first lady Laura Bush ever had "a real job".
Asked in an interview how she would differ from Mrs Bush as first lady, Mrs Heinz Kerry said: "Well, you know, I don't know Laura Bush. But she seems to be calm, and she has a sparkle in her eye, which is good. But I don't know that she's ever had a real job - I mean, since she's been grown up." In a statement put out later by the Kerry campaign, Mrs Heinz Kerry said she had forgotten Mrs Bush's work as a teacher and librarian. "I appreciate and honour Mrs Bush's service to the country as first lady, and am sincerely sorry I had not remembered her important work in the past," the Heinz Kerry's statement said.