Voting arrangements in Florida do not meet "basic international requirements" and could undermine the US election, former US President Jimmy Carter says.
Carter has monitored more than 50 elections worldwide
He said a repeat of the irregularities of the much-disputed 2000 election - which gave President George W Bush the narrowest of wins - "seems likely".
Mr Carter, a veteran observer of polls worldwide, also accused Florida's top election official of "bias".
His remarks come ahead of the first TV debate between Mr Bush and John Kerry.
They are expected to discuss the war on Iraq and homeland security during the programme on Thursday.
Both men have cut back on their campaign touring to go behind closed doors and rehearse the arguments and techniques they will need during a series of three debates to be held over two weeks.
Each has held mock debates with aides standing in for their opponent.
Tens of millions of television viewers are expected to watch Thursday's head-to-head.
Mr Kerry, a debating champion at high school and college, will hope it can help him claw back a deficit in the polls variously put between 3% and 9%.
In an article in the Washington Post newspaper, Mr Carter, a Democrat, said that he and ex-President Gerald Ford, a Republican, had been asked to draw up recommendations for changes after the last vote in Florida was marred by arguments over the counting of ballots.
Mr Carter said the reforms they came up with had still not been implemented.
He accused Florida Secretary of State Glenda Hood, a Republican, of trying to get the name of independent presidential candidate Ralph Nader included on the state ballot, knowing he might divert Democrat votes.
He also said: "A fumbling attempt has been made recently to disqualify 22,000 African Americans (likely Democrats), but only 61 Hispanics (likely
Republicans), as alleged felons."
Mr Carter said Florida Governor Jeb Bush - brother of the president - had "taken no steps to correct these departures from principles of fair and equal treatment or to prevent them in the future".
"It is unconscionable to perpetuate fraudulent or biased electoral practices in any nation," he added.
"With reforms unlikely at this late stage of the election, perhaps the only recourse will be to focus maximum public scrutiny
on the suspicious process in Florida."