“I am someone who just won’t quit,” shouted Hillary Clinton, and the flags waved, and the crowd cheered, and she nodded vigorously, in profound agreement with herself.

Mrs Clinton was on stage at a crossroads in a small town for one of her final rallies before today’s Pennsylvania primary. This could prove to be one of the last acts in her presidential campaign: but from appearances it might have been one of the first.

If this were a scene from Rocky, the boxing film about the local hero to whom she has often compared herself, this would be the moment when Sylvester Stallone is slumped bloodily against the ropes, eyes glazed, being fanned by men with broken noses urging him to get to his feet for one last round.

But it is not like that. Mrs Clinton seems as tough, determined and chilly as she has always been. “I have no doubt in my mind we can make this happen,” she said, hoarse from the repetition, but otherwise utterly unchanged. Not a hair is uncoiffed, not a thread on the oatmeal suit is out of place, hardly a word is any different, or any less convincing, than when this marathon began last January.

By the standards of any normal bout, Mrs Clinton should be flat on the canvas: Barack Obama, her rival for the Democratic nomination, has won 28 states, twice as many as her; he has garnered 13.3 million votes, 700,000 more than Mrs Clinton; he has raised $237 million (£120 million) to her $193 million. And, most importantly, he has amassed 1,635 delegates to the Democratic National Convention, to her 1,474.

Clinton campaign workers dole out T-shirts, badges and stickers. “Here, take a bunch; take as many as you want.” It is almost as if they suspect that, come tomorrow, there may be no more use for them, the “Hillary for President” icon an artefact of political history.

It is roastingly hot on the Tarmac, but from where I am standing, just a few feet from the stage, Mrs Clinton still seems utterly cool. She points to someone in the crowd, and opens her mouth wide in fake excited recognition. When the television footage is replayed, it will look as if Mrs Clinton has spotted old friends in the crowd, people who have come from far and wide to stand at her feet.

This is a Clinton trademark, a bit of fancy political footwork that she has used for nearly 20 years. She is such an experienced fighter that she barely knows she is doing it. But as I gaze up at her, battling away in a boiling little town, I realise something else about this extraordinary politician: she is actually enjoying this. After so much scandal, controversy and conflict, fighting with her back to the wall is her natural default position. She can even joke darkly about it. Asked what her supporters should say to voters on her behalf, she replied: “Oh, just knock on the door and say, ‘She is not as bad as you think’.”

Mrs Clinton may not float like a butterfly, but she has stung like a bee, with a series of excoriating television advertisements accusing her rival of accepting cash from lobbyists and corporations despite his claim to be independent of special interests.

If anyone has seemed to sag at the knees slightly in the last bruising hours of the Pennsylvania campaign, it has been Mr Obama. He knows that if he can deliver the knockout punch here, then even Rocky Clinton will not be able to get up again, but the sheer ferocity of the closing moments appears to have taken him by surprise. “Look, our campaign’s not perfect,” he told voters at one rally. “There have been times where, you know, if you get elbowed enough, eventually you start elbowing back.” The world of sharp elbows is where Mrs Clinton has lived for two decades.

The gap between the Mississippi primary on March 11 and today’s Pennsylvania contest has created the longest, most bitter campaign in any single state since the presidential contest began in Iowa on January 3.

Mrs Clinton can still pull off an upset, in the manner of Rocky Balboa. Her support among women, Hispanic and the white working-class remains solid. The Obama camp insists that anything less than a double-digit win for Mrs Clinton represents defeat. But if she makes a strong enough showing, most analysts believe that she will continue in the contest, claiming that only she can win the big states essential to presidential victory, and the fighters will stagger on to the next rounds in Indiana and North Carolina.

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