Barack Obama and Mitt Romney's campaigns are setting the stage for legal battles in key swing states, particularly Florida and Ohio, if the election result is close enough to be challenged in court.
Florida's Democratic party laid down a marker at the weekend with a lawsuit as electors once again grappled with long lines in parts of the state after officials circumvented new laws limiting early voting. The legal action, before any ballots have even been counted, accused the Republican-controlled state legislature of cutting short early voting, which has typically been used more by Democrats, in order to discourage opponents from casting ballots. The lawsuit is likely to form part of the basis of a court battle if the state proves decisive in putting Mitt Romney into the White House.
Romney must win Florida to stand a chance of taking the presidency. Some polls put him neck and neck with Obama in the state, although others give the Republican the edge.
If Romney takes Florida, the election is likely to be decided in Ohio, where the Republican secretary of state, Jon Husted, is facing accusations of attempting to manipulate the casting of provisional ballots, which tend to favour Democrats.
Independent monitors also warn that Ohio's system for issuing absentee ballots is failing to find registered voters and wrongly telling people they are not entitled to cast a ballot.
In Florida, some election administrators effectively extended early voting by other means in several places – including Miami, and Hillsborough County around Tampa – regarded as decisive in deciding who takes the state. They are permitting voters to pick up absentee ballots, fill them out on the spot and then deposit them.
"It's not early voting," said the Hillsborough county elections supervisor, Earl Lennard. "But it is voting early."
The Republican state governor, Rick Scott, has turned down appeals to extend early voting.
A former Democratic governor of Florida, Charlie Crist, chided Scott.
"It seems suppressive not to do so," he said as he campaigned on behalf of Obama in Tampa. "It's unconscionable to me that our governor would not sign an extension. This is a precious, sacred right that we have in our country. We're fortunate we get to choose our leaders. It seems to me that if you value that precious right you'd do everything to encourage people to vote. It seems suppressive not to do so. I just don't understand why someone would not extend it."
Wendy Weiser, head of the democracy programme at the Brennan Centre for Justice, which joined legal battles against attempts in various states to impose voter identification laws and other election requirements, said that the long lines to vote in Florida are a product of Republican machinations.
"This is a clear legacy of the effort to restrict voting this year," she said.
Weiser also warned that the "huge problem" of the absentee ballot system in Ohio could have an impact on the outcome of the election. About 1.3m ballots have been requested. She said there could be tens of thousands, possibly hundreds of thousands, of people wrongly told they cannot vote.
"That is a huge number of votes that will be affected," she said. "We may see the fight over this election move to the courtroom."
Weiser said there is also a problem with provision ballots in the state.
Democrats accuse Husted, the Republican secretary of state in Ohio, of making an eleventh hour bid to tamper with provisional ballots which are recorded when there's a complication with an individual's vote such as an address that doesn't match the voter roll.
More than 200,000 provisional ballots were cast in Ohio in 2008, most of them by Democratic voters who tend to be more transient and therefore get caught up in disputes over addresses.
On Friday, Husted issued an executive order requiring voters to fill out key parts of the registration forms previously done by poll workers.
"This was a clear effort to increase the statistical likelihood of there being errors that would justify disenfranchisement of provisional ballot voters who tend to be Democratic," said Subodh Chandra, a lawyer with a homeless coalition in north-east Ohio who identified the governor's move and has challenged the forms with a lawsuit. "It's yet another game he's playing to put impediments in the way of voters."
The court will rule on the dispute before 16 November – 10 days after election day when the provisional ballots will be counted.
In another swing state, Republicans have written to Matt Schultz, Iowa's secretary of state, alleging "what appears to be illegal activity involving Democratic and Obama campaign operatives".
"According to reputable media reports, Democratic operatives allegedly encouraged elderly persons to fill out and falsely sign absentee ballot requests for other family members. Additionally, the same reports suggest that Democratic operatives themselves may have even falsely filled out and submitted absentee ballot requests without the permission of the Iowa voters on whose behalf the requests were submitted," the letter said.
The Republicans said they "understand that the Iowa Department of Criminal Investigations is investigating this matter".
Chris McGreal is the Guardian's Washington correspondent. He has previously been posted in Johannesburg and in Jerusalem.