Hundreds of US government observers are being deployed across the country to ensure minority groups do not face discrimination during the election.
With four days to go, the election appears too close to call
About three times as many staff will monitor proceedings as in the 2000 presidential election, which saw a row over voter eligibility in Florida.
Florida officials are also sending new ballot papers to thousands of postal voters whose forms never reached them.
The candidates are targeting key states in the last days before Tuesday's vote.
John Kerry is scheduled to spend the day in Florida, one of the battleground states that could be won by either him or George W Bush and that may affect the result of the election.
Staff from the civil rights division of the justice department will monitor voting procedures in eight Florida counties, including those hit by claims of disenfranchisement and confusing procedures in 2000.
In total, 840 federal observers and more than 250 civil rights personnel will be at work in 25 states, including some like Pennsylvania and Ohio where the result is also expected to be close, the justice department announced.
They will be checking, among other things, that voters are not being challenged unfairly because of their race or language ability.
Large numbers of people have registered as new voters for this election - Republicans have concerns that some are fraudulent applications and Democrats want to ensure that their supporters are allowed to vote.
New ballot papers are being sent out in the heavily Democratic Broward county in Florida, though election officials there said initial fears that 58,000 forms had gone missing were inflated. But between 10,000 and 15,000 ballots are being resent.
High-profile supporters of both candidates will take the stage again on Friday to rally support for Mr Bush or Mr Kerry.
Former President Bill Clinton begins a three-state tour for Mr Kerry, while Hollywood star and California Governor Arnold Schwarzenegger will be with President Bush.
The president has also picked up the endorsement of Curt Schilling, pitcher with the World Series baseball champions - and Mr Kerry's hometown team - the Boston Red Sox, but the player cancelled campaign appearances on the advice of doctors.
Mr Bush and Mr Schwarzenegger will be in the Ohio state capital, Columbus, a day after Mr Kerry got what correspondents say was the largest crowd of his campaign when 80,000 people gathered as rocker Bruce Springsteen opened the rally for him.
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
The top issues in the candidates' speeches continue to be Iraq and each man's credentials as a leader.
Mr Bush said Mr Kerry's "willingness to trade principle for political convenience makes it clear that John Kerry is the wrong man for the wrong job at the wrong time".
He was supported by the military architect of the invasion of Iraq, now retired army Gen Tommy Franks who told Bush supporters that Mr Kerry "disrespects our troops".
Mr Kerry again focused on the controversy of the 350 metric tons of high explosive missing in Iraq, saying: "The president's shifting explanations and excuses and attacks on me demonstrate once again that this president believes the buck stops everywhere but with the president of the United States."
In other developments:
The New York Times reports that up to a fifth of presidential ballots could be cast before election day, which will affect where candidates are sent and money is spent during the final days of the campaign
- The latest Washington Post tracking poll suggests the vote is still too close to call, with Mr Bush predicted to poll 49% of the vote and Mr Kerry 48%
- Mr Kerry's running-mate, Senator John Edwards, accuses the Bush administration of favouring "special interest friends" after reports that the FBI is investigating the controversial award of Iraqi contracts to Halliburton, once run by Vice-President Dick Cheney.