America's presidential hopefuls are making their final appeals to Iowans to brave the cold and turn out to tonight's caucuses, an extraordinary test of on-the-ground organisation that will be crucial to victory in the first nominating contest of the 2008 election.

Beginning the day on the morning news shows, Barack Obama and John Edwards, the Democrats locked in the tight three-way battle with Hillary Clinton, had difficulty speaking, their voices hoarse after hundreds of appearances across Iowa in recent weeks. Mr Edwards, his eyes puffy, had just ended a non-stop, 36-hour bus tour of the state that culminated in a rally last night with the rocker John Mellencamp.

While all the candidates are spending the day fanning out across Iowa for one final time, their campaigns were already looking ahead to New Hampshire, which holds its primaries on January 8, in just six days time. Tomorrow morning all the main campaigns, and thousands of journalists, will land in the Granite State to begin intensive campaigning there, deserting Iowa, which has been lavished with their attention for months.

Both the Republican and Democratic races in Iowa are so close that nobody is sure if the results will settle anything. A third place finish for either Mr Obama and Mrs Clinton would deal significant blows to their campaigns, and probably a terminal one to Mr Edwards, who has been campaigning in Iowa for the past four years.

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Campaigners have spent the last few days making hundreds of thousands of phone calls to voters, urging them to turn out. Volunteers to Mitt Romney, the Republican, made 12,000 calls on Sunday alone. The campaigners have bought thousands of shovels to clear driveways, and vats of salt to de-ice walkways. Tens of thousands of volunteers are trudging through the snow going door-to-door, with lists of Iowans to target and lobby. Hillary Clinton has signed up 4,700 drivers to get her supporters to their precinct sites, and another 5,000 of her supporters are offering lifts. The core of her supporters are aged 55 and over. Elderly woman are particularly crucial to her hopes.

Mr Obama, who has drawn massive crowds in recent weeks, is trying to defy history by turning out a huge number of independents and young Iowans, a group that historically does not attend caucus night. He is also urging people who have never caucused before to turn out. If he succeeds, victory could well be his. Mr Obama, reflecting the biting cold, advised his volunteers to wear longjohns as they went door-to-door.

At one of his final rallies, before a crowd of 1,500 mostly young and cheering supporters, he also zeroed in on the key to his chances of victory. The pundits, he said, "say ah, those people. They're not going to show up. Students never show up." He added: "Are you going to prove them wrong?" The crowd erupted.

At a rally in Des Moines last night, his voice croaking, Mr Obama was still able to do basketball-style leaps to "high five" supporters leaning over barriers to touch him. But His wife, Michelle, could be seen telling friends "just one more day of this".

He told the rally that they were responsible for "the best grass-roots organisation Iowa has ever seen," before urging them on to "one last effort, one last test". A chorus of supporters behind him chanted: "We be-lieve! We be-lieve" and "Hope! Hope! Hope." Mr Obama gave the crowd what it wanted with a typically upbeat speech saying this was a once-in-a-lifetime opportunity for their generation to make "their mark on history". He compared those ready to caucus for him tonight to those who had "thrown off the yoke of the British Empire" as well as those who fought slavery in the Civil War or marched for Civil Rights, saying: "This is our moment. Listen to that voice in each of us that says there is something better, something to reach for."

A new poll this morning showed Mr Obama with a four point lead over Mr Edwards, with Mrs Clinton trailing in third. But a slew of other polls in recent days reflect a race that is effectively tied. An extraordinary number of Iowans - as many as 30 per cent - say they are still undecided, just hours before the 1,781 precinct sites open at 6.30pm (12.30am GMT). In a country of 300 million people, about 150,000 Democrats are expected to turn out in Iowa tonight - 0.05 per cent of the US population. Up to 90,000 Republicans are planning to caucus - 0.03 per cent of Americans. A victory for any candidate could generate such momentum that the choices of these Iowa voters could conceivably determine who will be the next president.

Joe Trippi, a senior adviser to Mr Edwards, said: "Turnout will be the big, big thing. If it's a record - way over the record - then the Obama campaign has succeeded in bringing in a lot of new voters. He will win. But if it's 135,000 to 145,000 we have got a good chance."

Mr Trippi predicts that if one of Mr Edwards or Mr Obama trails in third and is seen as "way off the pace", Mrs Clinton's opponents will coalesce around the other. "She wants the three to stay bunched. But if Clinton comes third, the story on Friday will not be about who has taken first - it will all be about her."

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