Carter has monitored more than 50 elections worldwide
Voting arrangements in Florida do not meet "basic international requirements" and could undermine the US election, former US President Jimmy Carter says.
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.
Recent opinion polls give Mr Bush a lead over Mr Kerry of between 3% and 9%.
In an article in the Washington Post newspaper, Mr Carter, a Democrat, wrote that reforms - agreed after the last vote in Florida had been marred by counting problems - had not yet been implemented.
It is unconscionable to perpetuate fraudulent or biased electoral practices in any nation
Shaky confidence in Florida vote
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.
"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."