The tune that accompanied Barack Obama’s arrival on Tuesday night at what was expected to be another victory rally said it all – it was Stevie Wonder’s “Signed, Sealed, Delivered, I’m Yours”. The senator’s organisers had clearly not had time to adjust the format following the shock announcement of Hillary Clinton’s win a few moments earlier.

Nobody, including the Clinton campaign, expected Mr Obama to be defeated in New Hampshire. But the reddest faces were among the polling agencies, who on Tuesday morning projected an Obama victory margin of up to 13 percentage points, and the media, which had all but written off Mrs Clinton’s prospects for achieving the Democratic nomination.

At her campaign’s victory event, which bore clear signs of having being planned as a concession rally, Clinton officials could barely conceal their pleasure at the media’s embarrassment. On the previous day, Bill Clinton had angrily alleged that there was a clear media bias against the former first lady and in favour of Mr Obama.

Pointing to cover stories such as this week’s “Weekly Standard” – “The Fall of the House of Clinton” – and the popular online Drudge Report, which maintained a “Clinton Death Watch” during the New Hampshire campaign, Clinton officials felt vindicated in the former president’s assessment. “So will the media now be announcing its withdrawal from the presidential race?” joked one Clinton official.

How did everyone, including the Clinton campaign, which was hoping until an hour or so before the result came out that it could contain Mr Obama’s margin of victory to within four or five percentage points, get it so badly wrong? The most widespread explanation is that everyone underestimated the degree to which Mrs Clinton succeeded in “humanising” herself in the eyes of the New Hampshire electorate.

In what was the first in a succession of emotional interludes during the intense four-day New Hampshire campaign, Mrs Clinton lost her cool during the Saturday night televised debate at Saint Anselm College in Manchester. Having been accused by John Edwards of embodying “the forces of the status quo”, Mrs Clinton angrily defended her “35-year record of change” and called on voters to take a “reality check” and dismiss the “false hopes” that her rivals were allegedly peddling.

It was Mrs Clinton’s first overt show of indignation in a campaign where until her surprisingly emphatic defeat in Iowa last week she played the part of the frontrunner who did not stoop to direct clashes with her rivals. Moments later, Mrs Clinton drew sympathetic applause from the audience when the moderator asked how she planned to overcome the fact that so many voters dislike her. “That hurts my feelings,” she said.

Then, on Monday, Mrs Clinton came close to tears at a gathering with voters in a diner when someone inquired where she found the stamina to carry on. The clip, which was endlessly replayed on the TV networks, was also derided by many commentators as having been contrived.

But Mrs Clinton’s rare display of vulnerability clearly left a different impression with many voters, in particular among women, who accounted for 57 per cent of Tuesday’s turnout in the Democratic primary and who broke in her favour by a margin of 47 per cent to Mr Obama’s 34 per cent. This reversed Mrs Clinton’s poor showing among women in Iowa. “That ‘choking-up’ moment was the turning point in the campaign,” said a senior Obama official.

There were other factors – largely beneath the radar – that pulled in Mrs Clinton’s favour. Of these, the Clinton campaign’s get-out-the-vote effort may have proved decisive in securing her narrow win. Stretching back to the 1992 campaign, the Clintons drew upon an extensive and formidable network of loyal unions, party grandees and grassroots activists to drum out voters across the small New England state.

Unlike the other campaigns, in particular those of Mr Obama and Mr Edwards, Mrs Clinton did not divert her New Hampshire resources to Iowa for the January 3 caucuses. “All the other campaigns pulled in their resources from around the country, including from New Hampshire, to make sure they won in Iowa,” said Jay Carson, Mrs Clinton’s spokesman. “We didn’t do that. So we were stronger than them on the ground in New Hampshire.”

Then there is the famed prickliness of New Hampshire’s citizenry, who are legendary for making their minds up late and going against conventional wisdom. That did not apply to Maureen Flaherty, a mother of six and grandmother of 10, who voted in a downtown precinct in Manchester on Tuesday afternoon, and who was not shy of admitting her preference. “Hillary talks a lot about healthcare and things that matter to us,” she said. “I like Barack Obama but I don’t usually follow these fashions.”