Image caption Known unknowns: Virgil Goode, Jill Stein and Gary Johnson

Third-party presidential candidates have held their own debate, chaired by television host Larry King. And as the polls suggest the race could be decided by a small number of votes in key states, could the "unknowns" make a difference?

The only two President Johnsons etched on American minds both made their mark on US history - Andrew impeached and Lyndon Baines signing off landmark civil rights legislation.

No-one thinks there will be a third in 2012, not even Gary Johnson himself, but the Libertarian Party candidate could yet be a factor in the outcome.

The former governor of New Mexico is the best known of the "other" 25 presidential candidates on the 6 November ballot, but that doesn't say a lot.

Johnson and Co struggle to make the national radar and are expected to hardly figure in the final tally come polling day.

Gary Johnson Former governor of New Mexico, 59

Libertarian Party

On the ballot in 43 states, despite numerous Republican legal challenges

Supports relaxed drugs laws, gay marriage, immediate Afghanistan withdrawal

Would abolish the Department of Education to help balance the budget Where Johnson stands on key issues Full list of presidential candidates

But in a contest that could boil down to fine margins in a couple of key battleground states, they could yet inflict a fatal wound on President Obama or his re-energised Republican challenger, Mitt Romney.

After all, two of the last five elections have featured significant contributions from a third candidate.

In 1992, millionaire Ross Perot damaged the incumbent George HW Bush and helped to propel Bill Clinton to the White House with only 43% of the popular vote. And Ralph Nader is still vilified by some Democrats for eight years later taking crucial votes away from Al Gore in Florida.

A sense of frustration that the potential Ralph Naders of 2012 have not had a voice prompted the Free and Equal Elections Foundation to organise Tuesday's debate.

Johnson, Virgil Goode of the Constitution Party, Jill Stein of the Green Party and Rocky Anderson of the Justice Party shared a platform in Chicago to discuss - among other issues - national security, government spending and drugs laws.

"It's obvious they're not going to win, but in the Constitution it never says there's a Democrat or a Republican Party," the moderator Larry King told Politico website. "It never mentions a two-party system."

Virgil Goode Former Virginia congressman, 66

Constitution Party

On the ballot in 38 states

Pledges to cut education and foreign aid, to balance the budget

Pro-life, defends gun ownership Where Goode stands on key issues

While Americans suffer a daily bombardment of opinion polls as election day looms, they are invariably asked to select from just two candidates - the rest are rarely included.

But Public Policy Polling (PPP) has surveyed the potential impact of the other candidates in key battleground states.

In Virginia, in a straight two-horse race, Obama is up three percentage points, says PPP's Dustin Ingalls. But add the third-party candidates Johnson, Goode and Stein, who are all on the ballot there, and Obama's lead increases to four percentage points.

"They might tip a very close race in the end back Obama's way. They might have an impact but it's unlikely to be a seismic one, although one point could be enough."

In Colorado, the addition of Johnson to the polling stretches Obama's lead from three to five points in PPP's poll.

"Typically, we've been finding Romney losing a little more support than Obama but not that much. In the main, it doesn't have a measurable impact on either candidate."

And it could be that as Romney has gained ground in recent weeks, his base support is less likely to switch allegiance to anyone else, says Ingalls.

"It's not surprising that third-party candidates don't get any more traction. There's a vicious cycle because they don't get a lot of attention therefore not much money and therefore not much impact or attention. It's a hard cycle to break."

Jill Stein Medical doctor, 62

Green Party

On ballot in 38 states

Pledges to provide 25 million green jobs and cut military spending

Would protect Medicare and Social Security

Arrested for staging a protest at being excluded from the presidential debates Where Stein stands on key issues

Johnson did have his moment in the spotlight on TV primetime last year when sharing a stage with the likes of Romney in the Republican primary elections, he cracked a joke that made headlines.

However, he was excluded from future debates and joined the Libertarian Party, with a manifesto that his adviser Ron Nielson describes as socially tolerant and fiscally conservative.

This kind of platform has wide appeal and he could reach his target of 5% of the popular vote, says Nielson, who wants people to back his man in protest at the dominance of the two-party system.

"Laws around the country make it impossible for a third-party candidate to get on the ballot. The rules are not the same for the Democratic or Republican candidate - the rules for signature gathering are 10 or 20 times more stringent than they are for the major parties."

In some states, you need about 750,000 valid signatures to get on the ballot, says Richard Winger, editor of Ballot Access News. But the rules on what is "valid" are so strict that the reality is double that.

In Maryland this year, thousands of signatures were ruled invalid because they did not contain the middle initial.

"Ordinary people who work can't spend hours and hours out on the street to persuade strangers to sign a petition, so they have to pay a group of people to do that."

That's about $125,000 before you even mount an advertising campaign, says Winger, who is in favour of proportional representation or the alternative vote system.

The 27 candidates (including Obama and Romney) on at least one state ballot is a record number, he says, which is an indication of the level of public unhappiness with the two parties.

Image caption Man in the middle: Perot with Clinton (left) and Bush

But even after overcoming the barriers and getting on to dozens of state ballots, the likes of Johnson, Goode and Stein are unknowns.

"Voters don't know them," says Larry Sabato, director of the Center for Politics at the University of Virginia.

"Virgil Goode is known in the district where he represented Congress for many years, Gary Johnson is known in New Mexico where he was governor and Jill Stein is known in her own household. And that's it.

"They haven't made any headway because when Americans are polarised they don't like a third-party candidate.

"They hate the other side so much they're not going to waste their vote. We've been trained to think of our system as two-party and we're wasting our vote if we cast it for another candidate. That's how most Americans think."