Tens of thousands of postal ballots have gone missing in the state of Florida, sparking fresh fears of irregularities in the US poll campaign.
The result of the 2000 polls was delayed by a recount in Florida
Authorities are investigating the apparent loss of 58,000 absentee forms in Broward County while officials have said replacements are being sent out.
Controversy over the vote in Florida in 2000 delayed the national result.
With five days until the poll, the presidential candidates are focusing their campaigns in crucial states.
The accepted wisdom is that whoever wins two out of the three states of Ohio, Pennsylvania and Florida will win the election, the BBC's Rob Watson in Washington says.
The latest opinion polls suggest that the race in all three is still fairly close, our correspondent adds.
For the fourth day, Mr Kerry held the president responsible on Thursday for 350 metric tons of explosive missing in Iraq.
"The commander-in-chief is not getting his job done," the Democrat said.
Mr Bush hit back, saying John Kerry was "the wrong man for the wrong job at the wrong time".
'No blame game'
The missing ballots have fuelled an atmosphere of intense suspicion in Florida, with Democrats already backing nine separate lawsuits in the state, says the BBC's Justin Webb in Washington.
KEY SWING STATES
1. Florida - 27 electoral votes
2. Pennsylvania - 21
3. Ohio - 20
4. Minnesota - 10
5. Wisconsin - 10
6. Iowa - 7
7. Nevada - 5
8. New Mexico - 5
9. New Hampshire - 4
If the outcome is close and decides the result in the presidential race - and both of those eventualities are perfectly possible - it seems virtually certain that protracted legal battles will follow, our correspondent says.
Electoral officials in Broward County say they will send more ballots after being overwhelmed by calls from would-be voters, unable to get to the polls, who fear that their votes have been stolen.
A police investigation into the missing ballots has not uncovered any indication of criminal wrongdoing.
The US postal service inspectorate said it was highly unlikely that 58,000 pieces of mail had just disappeared.
A spokesman said inspectors were trying to establish whether the ballots were ever delivered to the postal service.
Broward County elections supervisor Brenda Snipes said the situation was not a "blame game".
"What we're concentrating on is getting the ballots to the voter," she told the Miami Herald newspaper.
Campaigners are working to ensure that no-one loses the right to vote because of the missing ballots.
Democratic Party legislators have called on the electoral authorities to publish newspaper ads telling people of other options for voting.
Broward backed Al Gore in 2000
They also accused the Republican-dominated state government of dirty tricks.
Governor Jeb Bush - the brother of the incumbent president - has rejected suggestions of bias.
In 2000, Broward gave Al Gore his biggest margin among Florida counties. He won 67% of the vote there, while losing the state to George W Bush by 537 votes.
"It looks like they're trying to steal the vote again," said Diane Glasser, vice-chairwoman of the Florida Democratic Party.
Both the Democrats and Republicans have already begun filing lawsuits in states across the US, challenging different aspects of the election process.
Voting rights activists in Florida say the state could also face acute problems with vote-counting and the process of casting ballots in next week's election.
Early voting, which has been going on for more than a week, shows that new electronic voting machine technology is much slower than the old punch-card method, the BBC's Daniel Lak reports from Miami.
A high turnout could mean huge delays, our correspondent says.
Other concerns include accusations of voter intimidation by party representatives outside polling stations during the early voting.