Hillary Clinton could yet derail Barack Obama’s presidential campaign after video emerged yesterday of the former First Lady refusing to rule out a divisive floor vote at his nominating convention this month.

The tense behind-the-scenes battle over how to recognise Mrs Clinton’s achievements at the Democratic Convention, which opens in Denver on August 25, burst into public view with footage of her telling supporters that she was looking for a “strategy” for her delegates to be honoured.

Mrs Clinton, filmed talking to mainly women supporters in California last week, refused to rule out the prospect of including her name in an open ballot of all members of the convention. The move would be unprecedented for the leading losing primary candidate in recent times because it would force delegates to pledge publicly their support one by one.

“I believe we will come out stronger if people feel that their voices were heard and their views respected. That is a very big part of how we actually come out unified,” Mrs Clinton said. She received 18 million votes in the primary campaign, narrowly lost the pledged delegate race and registered a string of thumping victories over Mr Obama in the later stages of the nominating battle.

Mrs Clinton’s aides said it was highly unlikely that she would go as far as to demand a nomination roll call but the negotiations between the campaigns reflects the considerable bitterness that still exists, particularly with Bill Clinton and many of his wife’s supporters.

Mrs Clinton has been given the leading speaking role on the second evening of the convention — the night is effectively hers — but until late last night Mr Clinton’s role had yet to be determined. Faced with growing anger inside the Clinton camp of leaving a two-term president in limbo, Mr Obama personally offered a speaking slot to him hours before he departed for a week's holiday in Hawaii. It is understood Mr Clinton will make his speech before Mr Obama's vice-presidential pick addresses the convention.

Publicly, Mrs Clinton is doing much to help Mr Obama and is calling on her supporters to back him. She will campaign on his behalf in Nevada today, and in Florida just before the convention. After the video footage emerged, Mrs Clinton issued a statement saying that no decisions had been made on the convention, adding: “Senator Obama and I . . . will ensure that the voices of everyone who participated in this historic process are respected and our party is fully unified heading into the November election.”

Privately however, the Clintons remain sceptical that Mr Obama can win the White House. Although most voters will not focus properly on his race against John McCain until after the conventions have ended in early September, Mr Obama has stalled in the polls. A new survey also diagnosed a fresh concern for the Democrats — “Obama fatigue”. Nearly 50 per cent of voters feel that they have been “hearing too much” about Mr Obama.

The Clinton camp is also irritated by what they perceive as Mr Obama’s failure to work harder to help to pay off her campaign debt and to reach out to her supporters.

In Denver, diehard supporters are planning to demonstrate on her behalf, a prospect that unsettles the Obama camp. The groups include PUMA (Party Unity My Ass) and Just Say No Deal Coalition. David Axelrod, Mr Obama’s campaign manager, said: “We’re going to work this out.” He added that Mrs Clinton was “making a good-faith effort on behalf of Barack”, and the campaigns were talking every day.

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