Slowly, but surely, a note of realism is creeping into the chorus of defiance that has surrounded Hillary Clinton’s campaign for the Democratic presidential nomination.

For the moment she fights on, with tonight’s primary in West Virginia expected to deliver her a resounding victory in a state that Barack Obama has largely bypassed, except for a token visit yesterday.

However, Mrs Clinton and many of her leading supporters are said to recognise that the mathematical barrier – in the form of Mr Obama’s lengthening delegate lead – is now too high a hurdle for her to jump.

She has toned down her attacks on Mr Obama and now makes a point of promising that the party “will be united” – whatever the result of the nominating process – in November. Yesterday, even as her campaign chairman, Terry McAuliffe, promised “unequivocally” that Mrs Clinton would carry on until the final primary on June 3, she released a video message wistfully thanking her die-hard troops for their “sustaining support and friendship”.

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The awareness that Mr Obama is moving towards a seemingly inevitable victory is even sometimes reflected in verbal slips on the campaign trail. At one event in West Virginia, she described how the next president should address voters – “if he actually cares”. She then corrected herself, adding: “More likely, if she cares about it.”

Discussion is increasingly turning towards what role Mrs Clinton would play after her defeat. A close confidant of the former First Lady told The Timesthat Mr Obama was likely to be beaten by John McCain in November’s general election, saying: “So then there is 2012. She will only be 64 and that’s not too old to be president, is it?” Others are suggesting that her reasons for hanging on are more immediate. Although Mrs Clinton’s spokesman, Howard Wolfson, says that he has seen “no evidence of her interest” in being the vice-presidential nominee, there is still talk from within her camp of a so-called “fusion option”.

This worries Mr Obama’s team because it implies that she would have near-equal status on the Democratic ticket, and one of his leading backers, Senator Ted Kennedy, has begun pushing back against such a prospect. Mr Obama, he said crushingly, would be better off with someone who was “in tune with his appeal for the nobler aspirations of the American people”.

Some have suggested that Mrs Clinton is staying in the race in the hope that a series of strong performances in the final primaries can be used as leverage for help from Mr Obama on paying off her campaign debts of $20 million (£10 million) – which includes more than $11 million of her own money. David Axelrod, Mr Obama’s chief strategist, appeared to slam that door shut at the weekend, saying any idea that “she’s waiting for a cue or a signal from us or an offer of financial assistance” was demeaning to Mrs Clinton.

Another possibility was that she could be enticed into conceding defeat if she was promised a role leading legislation on healthcare – an issue close to her heart – through the Senate. However, Mrs Clinton is said to be reluctant to return to Capitol Hill because she feels bruised, even humiliated, by her failure to win more support among her colleagues in the Senate.

She has been angered by some of the scorn poured on her in recent days by the likes of Mr Kennedy, and there were reports yesterday that she was unlikely to heed calls for her to quit that have sometimes been laced with sexism. One source cited comments by Congressman Steve Cohen, who compared her to the murderous female stalker in the film Fatal Attraction, saying: “Glenn Close should have stayed in that tub.” The siege mentality within her inner circle is felt keenest by her husband, Bill, who spent much of his presidency defying those in Congress and the media who thought that he was finished.

He continues to campaign relentlessly for his wife, particularly in impoverished white rural areas, railing against the pundits who have declared that the race was over and who have “made fun” of the continued support for Mrs Clinton while “working people all over America have stuck with her”.

Mr Obama sought to emphasise the need for unity. “I’m honoured that some of you will support me. I understand that many more here in West Virginia will probably support Senator Clinton,” he said in Charleston.

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