Hillary Clinton’s presidential campaign forced a magazine to drop a negative story about her by threatening to cut off the publication’s access to the former President Bill Clinton, it emerged yesterday.

The ruthless response to GQ magazine, and its decision to bow to the ultimatum, reflects the enormous leverage Mr Clinton brings to his wife’s White House bid at a time when her quest for the Democratic nomination appears more formidable than ever.

The magazine, which is due to have Mr Clinton on its cover for its December issue, was told that the former President would no longer cooperate unless it pulled an article it was about to publish detailing infighting and tensions within Mrs Clinton’s campaign.

Despite protests at the magazine, the article was duly sidelined, according to a respected US political website. In an e-mail statement to The Times, Jim Nelson, the Editor of GQ, said: “I don’t really get into the inner workings of the magazine, but I can tell you that, yes, we did kill a Hillary piece. We kill pieces all the time for a variety of reasons.” He refused to elaborate.

The move by the Clinton campaign provides a graphic example of the be-hind-the-scenes hardball tactics it employs in keeping the New York senator’s relentlessly disciplined presidential bid on track and on message, and the power that she and her husband have in shaping how her White House bid is perceived.

On Sunday Mrs Clinton pulled off the rare feat of appearing on all five Sunday-morning political talk shows, a privilege the networks are unlikely to afford her rivals. Her lead in the national Democratic polls over her nearest rival, Barack Obama, is so big – almost 20 per cent – that pundits are now asking not if she can win the nomination, but if she can be stopped.

President Bush also thinks that she will win the nomination, it emerged yesterday, and has even indicated in private that he believes she will succeed him. White House aides, on Mr Bush’s instructions, have been privately briefing her – and other Democrat candidates – about Iraq in case she wins the election next November. They have been urging her not to commit to an immediate withdrawal if she takes office in January 2009, because Mr Bush wants his successor – Democrat or Republican – to continue prosecuting the war after he leaves the Oval Office.

Although Mr Bush often says that he will not handicap elections, he told the author of a new biography about him that Mrs Clinton has “got a great national presence, and this is becoming a national primary”. In an off-the-record session with broadcast journalists just over a week ago, Mr Bush, according to those in the room, gave the impression that he thought she would win the presidency and that he had been thinking about how to turn Iraq over to her.

Mrs Clinton’s lead in national polls, and similarly big leads over Mr Obama in the early primary states of New Hampshire and South Carolina, means that the contest in the first nominating state of Iowa has become crucial.

With Mrs Clinton succeeding in making her nomination look almost inevitable, her rivals are expected to be more aggressive and critical in a Democratic debate tomorrow night in New Hampshire, aware that time is running out to derail her.

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