The smooth road to the White House down which Hillary Clinton has sped so far threatens to become rocky in the Granite State of New Hampshire.

This is where the first presidential primary elections will be held in January, shortly after the Iowa caucuses, where Mrs Clinton is in a three-way fight with Barack Obama and John Edwards, her chief rivals for the Democratic nomination.

An opinion poll this week giving her a 23-point lead over Mr Obama suggests that she should easily win New Hampshire. But it is hard to find anyone who believes that the election will stay one-sided, particularly in a state that takes its role in picking the next president so seriously.

Ray Buckley, the chairman of New Hampshire Democrats, said: “There is going to be surprise – there always is. Someone leading now in the polls will not necessarily be leading in January.”

Fergus Cullen, his counterpart in the Republicans, agrees. “The Democratic race is about to become more competitive here. People want to see a contest, they want to see candidates tested. They don’t like the idea that one of them is walking away with it.”

New Hampshire and Iowa jealously guard their status as the “first in the nation” to choose their presidential candidates. Longstanding resentment at the influence exercised by these states – regarded as small, white and a bit too contrary – has prompted others to move the dates for their primaries forward to January or early February.

There were rumours of Mrs Clinton and Rudy Giuliani, the front-runner for the Republican nomination, employing a “national strategy”. With this they would have bypassed Iowa and New Hampshire to concentrate on the delegate-rich battlegrounds of Florida, New York and California, where their deep pockets and superior organisation would crush any insurgency coming from the early states.

But such talk has largely fallen silent in recent weeks with the dawning realisation that the changes to the electoral calendar may have had the unintended consequence of making Iowa and New Hampshire more, rather than less, important.

Mr Cullen said: “There is no longer any margin for error for a candidate who stumbles here.” Whereas Walter Mondale still won the Democratic nomination in 1984 because he had “five weeks to recover” from a shock defeat at the hands of Gary Hart in New Hampshire, next January’s primary “will set the tempo” for the states following immediately after.

Mr Cullen suggested that while candidates such as Mr Giuliani are essentially skipping Iowa, a front-runner “cannot afford to lose New Hampshire as well”. As they like to say in this state: “Iowans pick corn. We pick presidents.” Mr Obama and Mr Edwards are counting on a good performance in Iowa to ignite their campaigns and put real pressure on Mrs Clinton in New Hampshire.

Jim Margolis, a senior strategist in the Obama campaign, said: “If Hillary wins Iowa by 15 points it will be very difficult to come back. But if she loses or even wins by a narrow margin then New Hampshire becomes a huge problem for her. The idea that Hillary is unstoppable? That’s just crazy – it’s nonsense.”

Joe Trippi, a campaign strategist for Mr Edwards, knows how swiftly a front-runner can implode in Iowa and New Hampshire having worked for Howard Dean in 2004 when a seemingly unassailable lead swiftly disintegrated. “Hillary has a problem being out in front for so long. People in New Hampshire like to see someone like that being challenged.”

A TV debate here on Wednesday night for Democratic presidential candidates gave her rivals, trailing far behind in polls nationally and locally, at least some cause for hope that the Clinton campaign juggernaut can still be slowed, if not halted. In contrast to her commanding performances in previous debates, she appeared uneasy and brittle in the face of even the fairly tentative attacks made against her.

Mr Edwards led the charge, accusing Mrs Clinton of supporting “a continuation of the war” in Iraq and criticising her vote in the Senate this week for a resolution that could be the “first step to war with Iran”.

Mr Obama, who has often ducked confrontations with Mrs Clinton, followed in a slightly more diffident fashion by highlighting her remark that she had fought a “lonely” battle for healthcare reform in the 1990s.

“Part of the reason it was lonely, Hillary, was because you closed the door to a lot of potential allies in that process,” Mr Obama said.

Concerns about returning to the polarising era of her husband’s presidency were also raised by some of the long-shot candidates. Joe Biden said that it would be harder for Mrs Clinton to overcome hostility towards her among Republicans voters.

“I’m not suggesting it’s Hillary’s fault. I think it’s a reality,” he said, adding that the Clinton name inevitably dredged up “old stuff”. When it was pointed out that her position, ruling out the use of torture on a terrorist who had information about a “big bomb”, differed from that of her husband, she was forced to demonstrate her independence.

“Well, he’s not standing here right now,” she said, before adding: “I’ll talk to him later.”

Dante Scala, a politics professor at the University of New Hampshire, said that Mrs Clinton had done a good job at neutralising any threats of insurgency against her position in the state, employing experienced ground operatives and emphasising that she stands for change, as well as experience.

But he said that polls in New Hampshire “have to be taken with a whole shaker of salt – voters are only just beginning to take this seriously and most have not made up their minds”.

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