Democrat hopeful is ruthlessly exploiting working-class roots to fight for the votes in Pennsylvania.

"This is me in Scranton," says the voice of Hillary Clinton over grainy black and white footage of a beaming little girl in a white 1950s dress. "This is where my father was raised and my grandfather worked in the lace mill."

The camera cuts to a shot of a clapboard house, the rustic cottage in the Pennsylvania hills near Scranton where the Rodham family spent every summer. "There was no heat or indoor shower, just the joy of family," says Mrs Clinton's voiceover, as the music rises to a crescendo.

"I was raised on the American dream . . . we all need to dream it again and I promise we will."

The television advertisement, currently being played and replayed throughout Pennsylvania, could hardly be more schmaltzy, more obvious, or more effective: this is Mrs Clinton's latest, and possibly last, roll of the dice in her campaign to secure the Democratic nomination.

If Mrs Clinton can persuade enough white, working-class Pennsylvanians that she is one of them, and win tomorrow's primary by a substantial margin, then she may regain the momentum she needs to stay in the race. If she loses to Barack Obama, or fails to win convincingly enough, Mrs Clinton is sunk.

As the drawn-out Democratic battle heads into what may be its clinching contest, Mr Obama is also pouring money, time and energy into the state, outspending his opponent by a factor of nearly three to one. While Mr Obama is popular in the large cities, among liberals, blacks and students, Mrs Clinton is backed by older, whiter voters, conservative Democrats, Roman Catholics, and the blue-collar workers who make up the bedrock of the party's support.

Mrs Clinton's future depends on rediscovering her past, opening up the family photograph album and displaying her working-class credentials. At every campaign stop she stresses her links to this gritty, hardscrabble coal town in northeast Pennsylvania, where her father was born and lies buried.

"My feelings about Pennsylvania are real personal," she told a wildly cheering crowd on one of her last campaign rallies before tomorrow's vote. "My grandfather worked in the factory from the age of eleven . . . I have roots in Pennsylvania."

This may sound like an American version of the Monty Python sketch ("We 'ad it tough . . .") and it may be hard to swallow from a woman wearing an expensive suit who was brought up in privilege and comfort in Chicago, and who has earned $100 million with her husband since he left office. But it is also true, and goes down a storm with her Pennsylvania supporters.

Much has been made of the extraordinary Obama story, but in many ways the journey of the Rodham family is just as remarkable and, for many Pennsylvanians, comfortingly familiar.

Jonathan Rodham, Mrs Clinton's great-grandfather, was a Welsh miner who arrived in Scranton (on a train coach labelled "immigrants") in 1886 with his eight children, determined, like thousands of others fleeing European poverty, to extract a better life from the newly discovered anthracite fields. He made a living as a mine foreman, though never a fortune. His son, Hillary's grandfather, worked for 55 years in a lace textile factory.

Scranton was then a cauldron of America's industrial revolution, a place of belching factories, Satanic mills, train yards and, above all, mines. The tonnes of coal and anthracite carved out of the mountains fed the trains and factory furnaces: Hillary Clinton recalled, as a child, marvelling at the Lackawanna River running through the city, black with coal dust, and the burning piles on the horizon.

Hugh Rodham, Hillary's father, hopped a freight train to Chicago, founded a curtain business after the war and began to pull his family towards prosperity. The coal began to run out at about the same time, and the area has been in slow but steady decline ever since.

But Hugh Rodham never let his children forget where they had come from. The family spent every summer in the wooden cabin on Lake Winola, near Scranton, which Rodham had built with his father in 1921. Here the young Hillary learnt to shoot, to fish and to play pinochle, the American card game, with local men with names such as "Old Hank".

The countryside surrounding the town is strongly reminiscent of the 1978 film The Deer Hunter (the film was set in Clairton, Pennsylvania, to the south); it is a place where industrial life and rural ways collide and combine. The wooden cottage with the porch on Lake Winola still belongs to the Rodhams and Mrs Clinton repeatedly refers to it on the campaign trail: "A big part of my early experiences . . . it meant the world to me."

Mining this valuable seam of her past has paid political dividends. "She's tough. That's a real Scranton trait. That's an anthracite trait," says Christopher Doherty, the mayor of Scranton.

In Lake Winola, the Clinton connection is nurtured as a local treasure. "They are the nicest people, Regular Joe people," said Barb Jenkins, propping up the bar of the Blue Pelican, the town's only tavern, just a few yards from the door of the house known as "Rodham's View".

Ms Jenkins insists: "Everyone has had turmoil in their lives, and so has she, but she still walks out with her head up."

Mr Obama's ill-judged remarks about small-town voters who are "bitter" about losing jobs and who "cling to guns or religion or antipathy to people who aren't like them" have served to reinforce Mrs Clinton's appeal among the gun-owning, churchgoing inhabitants of towns like Scranton and Winola. Some 46 per cent of Pennsylvania voters own a gun, and almost one third of the population is Catholic.

Few will admit to feeling "bitter" about an economy that has seen the state's manufacturing sector lose one fifth of its jobs in just eight years, but many plainly are. "This area's really hurting," declares Barb Jenkins, ordering another Coors beer from the barman of the Blue Pelican.

Race is another potential factor in the Democratic contest. Polls show that Mrs Clinton is favoured by 59 per cent of white voters, compared with 34 per cent for Mr Obama. "There are plenty people, older folks mostly, who wouldn't vote for a black man," says Ms Jenkins. Only 56 per cent of Clinton supporters in Pennsylvania say they will vote for Mr Obama if he wins the nomination.

Mr Obama is leading in the national race for delegates, with 1,645 to 1,504 for Mrs Clinton. Roughly 2,025 are needed to secure the nomination, putting the focus firmly on the nearly 800 superdelegates, party leaders and officials with an automatic vote at the Democratic National Convention.

If Mrs Clinton fails to win Pennsylvania handsomely, those superdelegates are certain to start drifting into the Obama camp; if, however, she chalks up another resounding "come-back" victory, her team will argue that she, and only she, has the broad appeal to win the big swing states in a presidential battle against the Republican John McCain in November.

Mrs Clinton returns to Scranton today to rally her troops on what, over the past few weeks, has become home turf. On the way she stopped at York, in central Pennsylvania, an old colonial town that was the scene of some of the fiercest battles of the Revolutionary and Civil Wars.

"Madam! . . . President! . . . Madam! . . . President!" chanted the crowd. "We're not bitter, just voting better," read the placards; "Make History" and "We've got your back, Hillary". Yet no amount of noise can obscure the fact that Mrs Clinton has her back to the wall.

"I have no doubt in my mind we can make this happen," she declared, having described, once more, her sense of belonging to Pennsylvania. "I think there's a historical pattern here. It took a Clinton to clear up after a Bush - it's going to take another Clinton to clear up after another Bush."

Yet her remarks also carried the faint whiff of finality, a sense that after one of the longest, toughest and most fascinating fights in modern electoral history, the decisive battle is at hand. She has begun to speak of the duel with Mr Obama in the past tense: "This has been a vigorous contest. It has been a privilege and an honour."

Having emphasised historical precedent, and her own past, Mrs Clinton may also reflect on the history of York itself. During the Civil War the town famously surrendered to the Confederates, although a young Union officer, the "boy general" George Custer, valiantly held up part of the advancing army.

Pennsylvania was the site of Custer's First Stand. It may be the setting for Hillary Clinton's last.

By Ben Macintyre, The Times, April 21, 2008