While much of the attention in the US election focuses on the daily verbal clashes between John McCain and Barack Obama, a battle is being fought with as much intensity on the ground involving tens of thousands of lawyers and campaign staff.

In courtrooms in Ohio, Indiana, Louisiana, Georgia, Michigan, Florida, Wisconsin and other states, lawyers representing Democrats and civil liberties groups are locked in fights with Republicans over who is allowed to vote.

The Republicans say they are only trying to prevent voter fraud. The Democrats, burnt by what happened in Florida in 2000 and Ohio in 2004, say they are trying to ensure the Republicans do not steal the election by making it difficult for poor whites, African-Americans, the young and other potential Democratic voters to cast their ballots.

If Obama is heading for a landslide as the polls suggest, the issue could be academic in terms of the final result. But if the poll gap narrows over the next few weeks it could be critical. An academic study of the 2000 vote estimated that 2 million people were not allowed to cast their ballots, more than those who lost their votes through the much-publicised faulty voting machines.

Laughlin McDonald, who takes the lead on voting rights for the American Civil Liberties Union, which is at the forefront of the registration battles, said yesterday it was possible that registration challenges by Republicans could match or outstrip those in 2000 and 2004. The increased interest in the election because of Obama on the part of African-Americans and the young could result in the biggest turnout in US election history and also increase the scope for voter suppression.

"There always is a problem with voter registration but we are seeing more of it than in the past ... I can only say we've gotten more calls and are investigating more cases than before," McDonald, author of several books on voting rights, said.

Unlike most of Europe, where the mechanics of voting is not in party hands, the US process is left to state governments, and the parties in power often exploit this advantage. Democrats claim that rules introduced since 2004 in Republican-run Georgia, Indiana and Florida require specific pieces of ID that are unnecessary given there is little recent history of double-voting. They say the changes are politically motivated, aimed at African-Americans, Latinos, the young and other groups that tend to vote Democrat.

Indiana and Georgia are asking for driving licences that include photographs, but that could discriminate against people without cars. A recent study found that while 80% of Americans have cars, only 22% of African-Americans do.

There is the prospect on election day, as a result of the heightened interest, of huge queues, and this could be exacerbated by Republican activists mounting challenges over IDs. Lengthy queues could dissuade some from staying to vote. In 2004 some people had to queue in African-American neighbourhoods for two to three hours in Ohio, a state that Bush won by a slim margin.

There have been several successful legal challenges in Ohio this year, including one last month that overturned a 2006 law that required a piece of registered mail to be sent to every voter in the state. Mail was not forwarded and anyone whose name appeared on the list as mail returned could be challenged on voting day, a process known as cageing. If it had been allowed to stand an estimated 600,000 people would have been disenfranchised.

Professor Daniel Tokaji, of Ohio State University's Moritz college of law, who specialises in election law and has been involved in legal actions in defence of voter rights, said: "There is a major systemic problem in the US. The people running the election have a stake in the election. It is the quintessential problem. If you had to design a system from scratch, you would not do this."

He added: "There are occasional examples of suppression involving Republican-leaning voters but, in the main, it is Democratic-leaning voters that are targeted."

After another successful challenge that Tokaji was involved in, voters in Ohio who wanted to vote early were allowed to register and vote on the same day. The state Republican party had gone to court to block same day registration and voting.

In Wisconsin a judge is to rule on October 23 on a lawsuit filed in August by the Republican attorney general, John Byron Van Hollen, seeking to force ID checks on 240,000 to 1 million voters in the state.

Last month in Florida the registration applications of more than 5,000 people were held up on identity grounds. Under Florida law the name on a driving licence has to perfectly match that on the government's databank of social security numbers and names. In fact there are many spelling mistakes and mismatches, particularly in the Latino community, who sometimes use shortened versions of their names on social security cards and revert to their longer family names for driving licences. Even minor differences can provoke challenges.

In Florida's Orange County more than 50% of the 672 challenged registrations were Democrats and only 10% Republicans. The Republicans say the discrepancy is because of the much bigger Democratic registration drive this year.

One of the big demographic groups being targeted in states across the US is university students, among whom Obama enjoys support over McCain of at least two to one. Given potential confusion over whether they vote in the state in which they study or are from originally, there is a danger many will find themselves purged from voter lists.