In a chilly barn, surrounded by supporters perched on bales of straw and with the smell of pig excrement in the air, John Edwards is making his pitch to be the next President of the United States.

“You don’t make a hog fatter by weighing it,” he says to knowing nods as he explains – hands thrust into the back pockets of his faded jeans – why testing in schools does not make children any cleverer. Casey, the dog at the Rosman family farm in Harlan, Iowa, stirs briefly at the mention of hogs and settles back to sleep.

A couple of hundred miles east across this bleak mid-western landscape, Barack Obama has – as usual – attracted a bigger crowd at a larger barn, this time decorated with seasonal pumpkins and fairy lights.

He combines a ritual appeal to local farmers – “what is good for rural America will be good for the rest of America” – with his distinctive brand of urban chic. “I want to make government cool again,” he says.

The two main rivals to Hillary Clinton for the 2008 Democratic presidential nomination are making defiant stands in Iowa, traditionally the first state to pick a candidate.

National opinion polls suggest that Mrs Clinton, with a lead of more than 30 points, is heading for a coronation.

But Iowa remains locked in a tight three-way contest. Mr Obama and Mr Edwards hope that a victory will be the springboard to turn Mrs Clinton’s lead on its head.

If they cannot stop her here, goes the logic, they cannot stop her anywhere. David Plouffe, Mr Obama’s campaign manager, said recently: “Iowa – that’s the whole shebang.”

For Mr Edwards, a distant third in national polls, Iowa is perhaps even more crucial. Asked what would happen if he lost the state, he replies, with a shrug: “The same thing that happens to any campaign.”

The two pretenders to Mrs Clinton’s throne are, however, playing Iowa’s peculiar politics in very different ways. The state does not have an election but a caucus system with voters gathering in 1,784 different venues to reach a consensus on which candidate to back. In 2004 just 124,331 people took part in the Democratic caucus, just 6 per cent of those registered to vote.

Mr Obama hopes to expand this selectorate beyond Democrat activists by persuading young people – among whom he generates huge enthusiasm – and independents to take part in January. His core campaign message is that he can “bring America together again” and he has in general adopted a measured, cerebral approach even to his increasingly sharp criticisms of Mrs Clinton.

“I think she is a very capable person. She is sincere, she is tough, she cares about the Democratic party’s values,” he tells a meeting in Tipton, Iowa.

“But I also think that what is needed now is not what she has.” He dislikes Mrs Clinton’s “conventional approach” to foreign policy – including Iraq and Iran – and asks people to take “a little more of a risk with me”.

Mr Obama says: “She is a known commodity, Bill [Clinton] is there, we know pretty much what we’re going to get – but that’s the problem – we know what we’re going to get.” The Obama campaign is exceptionally well-organised in Iowa with 31 district offices. Aides insist they have learnt the lessons of Howard Dean – another Democratic candidate who tried to galvanise young voters – in 2004.

Mr Edwards, a veteran of that contest, is putting his faith in the traditional caucus-goers whom he has relentlessly courted over the past three years and describes as the “guardians” of the US presidency.

He is much shriller in his criticisms of Mrs Clinton and makes an old-fash-ioned populist appeal for support. Emphasising that “I’m the only candidate who grew up in rural America”, he evokes images of the “shared prosperity” in the 1950s and lambasts the Clinton White House for introducing free-trade deals which hurt farmers.

At meetings in western Iowa this week Mr Edwards is introduced by Ben Jones, a former congressman who played the character of Cooter in the TV series the Dukes of Hazzard.

The presence of “Cooter” aides say, embodies the “electability argument”, which has been roughly translated as saying that as a white male – and a southerner – Mr Edwards is the only top-tier Democrat who can win the White House from the Republicans.

Sometimes Mr Edwards appears to be trying just a little too hard. When he tries the weighing hogs line at a high school meeting, he is greeted with silence from children who, presumably, don’t like being compared to fattened pigs.

“C’mon,” he implores, “this is Iowa!”

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