The Democratic and Republican parties have warned that it could take weeks to decide the Nov. 2 election and have hired rival armies of attorneys and observers for the battle.
With US President George W. Bush and Democratic Senator John Kerry running neck-and-neck in opinion polls, both camps fear a repeat of the 2000 election debacle in Florida.
Kerry said on Sunday that he had put together a legal "dream team" to protect voter rights.
"If it is a close election in any one state, it may be days or weeks before we know who the actual winner is," warned Tom Josefiak, general counsel for President Bush's campaign.
In Florida four years ago, a controversy erupted over punch-card ballots that often failed to clearly show which candidate's name was punched out. The confusion left the US without a clear election winner for 36 days until the Supreme Court, in a controversial 5 to 4 decision, halted a recount of Florida ballots.
Bush beat vice president Al Gore by 537 votes and the Florida result secured the presidency.
There are growing fears that the winner between Bush and Kerry will also not be announced on election night.
Project Vote, a non-partisan organization, predicts a litany of lawsuits, notably regarding "provisional" ballots that allow people who are not sure whether they are registered to cast a vote, leaving authorities with the task of deciding whether the vote should be counted.
Legal challenges could be filed against the accuracy of electronic voting machines that do not provide papers that could be used for a possible recount.
The Republican and Democratic parties will deploy tens of thousands of observers to monitor the election across the country and discourage attempts by both camps to intimidate voters.
On Sunday, Kerry said at an African American Baptist church in Ohio that he had put together a "voter protection dream team" of top lawyers to forestall vote fraud and disenfranchisement.
"Let me just make clear to you, we are not going to let this be a repeat of 2000, we are not going to see a million African Americans deprived of their votes in America," he said.
Lawsuits contesting ballots, voter registration and voting machines have already been filed.
Elliot Mincberg, legal director of People for the American Way Foundation, said the early law-suits may help avoid post-poll challenges.
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