Final results in the 2000 presidential race were delayed for 36 days due to disputes over balloting in Florida, and resolved only after US Supreme Court intervention.

"If it stays at ... basically a 50-50 split, I don't see any way in the world that we're going to know who the president is, maybe for weeks and goodness knows how much longer," said Ralph Neas president of the People for the American Way foundation, which is participating in a massive election monitoring exercise.

"I think we'll have huge issues in many states, Florida is obviously ground zero, although I worry a lot about Ohio," said Barbara Arnwine, director of the Lawyers Committee under Civil Law, which is also participating in the Election Protection organisation.

"I believe that on Wednesday morning there will be a huge contest and fight over provisional ballots," she said in reference to the ballots cast by voters whose eligibility is challenged.

A report published by the two groups and the National Association for the Advancement of Colored People (NAACP) on Monday claims that Republicans had a strategy to launch "last-minute challenges to voter registrations by the tens of thousands in several states."