IN the state of New Hampshire, where Hillary Clinton is locked in a neck-and-neck struggle with Barack Obama, her rival for the Democratic presidential nomination, the Clintons learnt long ago that the race for the White House is not to the swiftest, but to the candidates with the staying power and the grit to slog to the finish, through embarrassments and reversals.

The Clintons were visiting an organic yoghurt factory during Bill's 1992 White House campaign. His affair with Gennifer Flowers, a nightclub singer, had already been revealed when suddenly an old letter emerged exposing him as a Vietnam draft dodger.

He went into the cooler room, lay down on the floor, stared at the ceiling and thought, "It's over." But Hillary stayed in control. "She really carried the ball," said Meg Hirschberg, the wife of the factory owner. "He was very upset and distracted, but she smoothed everything over. She's tough." Bill Clinton went on to crown himself "the comeback kid", even though he only came in second in New Hampshire. As his wife's campaign stumbles and loses momentum, her team are recalling how Bill got through his near-death experience to create the new legend of Hillary, the "comeback girl".

The bar is being set so low that insiders say she would have to lose three of the earliest voting states to Obama ? Iowa, New Hampshire and South Carolina ? before becoming seriously rattled. One or two losses would not be enough.

"The Clintons are famous for having the drive to keep going through the toughest crises and constantly fighting against the odds," said Leon Panetta, Bill Clinton's former chief of staff. "If you don't have that fire in the belly, you're not going to make it."

Karl Rove, the architect of President George W Bush's victories, last week described the "longest leadership selection contest in the democratic world" as a mess. "Next fall we'll elect a president who's spent two years rocketing around the country in an aluminium tube and sleeping in strange hotel rooms on a brutal, exhausting campaign trail," Rove wrote in The Wall Street Journal.

The marathon endurance test has left its mark on Hillary Clinton's 60-year-old face. A picture on the Drudge Report website last week showed her with the bags and crow's-feet of a woman who has spent the past year drumming up votes and fundraising coast to coast, prompting Rush Limbaugh, the conservative radio host, to ask, "Will this country want to actually watch a woman get older before their eyes on a daily basis?"

Women age, men mature, Limbaugh suggested unkindly. As if to confirm this thesis John McCain, the 71-year-old Arizona senator who describes himself as "older than dirt, with more scars than Frankenstein", has reemerged as a potential Republican winner, even though his campaign collapsed in acrimony in the summer. His resurrection suggests that the long-running primary season has the merit of allowing voters to kick the tyres of all the candidates before declaring a preference.

Tom Edmonds, a Republican consultant who predicted McCain's demise last summer, said: "I can see a scenario where the guy I beat up on could be a safe haven for Republican voters." Yet last week brought a last-minute surprise for the tried and tested McCain, who was accused of doing legislative favours for a female lobbyist, a charge that he vehemently denies.

As the campaigns enter the final stretch, dirt has been flying liberally. In the National Enquirer last week John Edwards, the Democratic candidate, was accused of impregnating Rielle Hunter, a video film-maker who worked on his campaign.

Hillary Clinton boasted in Iowa that there are no skeletons left in her cupboard. "I've been vetted. I have been in the political arena in our country very intensely for 16 years," she said. But Obama has also been tested ? ironically by Clinton's camp.

His ability to cope with the stresses of the campaign, despite efforts by Clinton's supporters to draw attention to his youthful cocaine use and Muslim roots, has been one of the strongest recommendations in his favour.

When Obama began running, he looked exhausted as he struggled to cope with the rig-ours of life on the road, his Senate responsibilities and duty towards his young family. As he tells it, his campaign chant "Fired up, ready to go" comes from a robust lady in South Carolina who inspired him when he was dog-tired and miserable.

Yet it is Clinton who looks lined now and Obama who is on a roll. Betsy Myers, a senior member of his campaign, said: "Over the summer it was tough. The polls were tough and the press were tough, but Barack held steady. He's never given up and never let go and that bodes well for his leadership style."

Hirschberg, who witnessed Clinton's composure 15 years ago, said last week she was backing Obama: "I've been impressed by the way he's conducted himself on the campaign trail. In the past couple of weeks the Clintons have bared their fangs. It's a side of them that is really unsavoury."

Panetta believes that Clinton has the power to crush Obama's challenge. "She clearly has had some setbacks and Obama's giving her a real race. The campaign has shown him to be a very credible candidate, but it is testing his ability to sustain his momentum," Panetta said.

"The most important thing Hillary has to do is hold steady and not look as though she is panicking in any way."

By Sarah Baxter, The Sunday Times, December 23, 2007