Barack Obama, the presumptive Democratic nominee, wrote out a $4,600 cheque for his vanquished rival, Hillary Clinton, last night as he tried to persuade her major financial backers to switch their support to him.

Mr Obama and Mrs Clinton plan to campaign together today in Unity, New Hampshire, in what will be their first public appearance together since the former First Lady bowed out of the race three weeks ago.

But the first step in a two-day push to unify the party came at a Washington hotel last night when the Illinois senator met with more than 200 of the biggest Clinton fundraisers.

Behind the scenes, the two sides are still negotiating on the shape of Mrs Clinton's future involvement with the Obama campaign. Although it now seems clear that she will not be his running mate, Mrs Clinton could prove invaluable in delivering blue-collar white and Latino voters to Mr Obama.

She told her donors last night that they must make electing Mr Obama a priority, although she acknowledged that hard feelings remain from their hard-fought primary battle. “But we are a family, and we have an opportunity now to really demonstrate clearly we do know what’s at stake, and we will do whatever it takes to try to win back this White House,” Mrs Clinton said.

In return, Mr Obama announced that he will help pay off Mrs Clinton’s campaign debt of more than $20 million campaign debt, personally writing a cheque for $4,600 – the maximum allowed. His announcement that he will urge his donors to help pay Mrs Clinton’s debts brought a standing ovation.

Some Clinton donors had been frustrated that the Democratic nominee-in-waiting had not done more to help her pay the bills even as they are expected to help fund his campaign. “I’m going to need Hillary by my side campaigning during his election, and I’m going to need all of you,” Mr Obama told them.

He recounted how he had told his top fundraisers this week “to get out their checkbooks and start working to make sure Senator Clinton – the debt that’s out there needs to be taken care of.”

Mrs Clinton’s debt includes $12 million of her own money lent to her campaign – and she has said that she is not asking for help paying that back.

Three Clinton confidants – Cheryl Mills, Minyon Moore and Robert Barnett – are in talks with the Obama campaign to work out details of her future involvement, including travel, her role at the national convention and resolution of her debt.

Part of their argument has been that Mrs Clinton can spend more time helping Mr Obama if she isn’t raising money to pay her bills.

Meanwhile, Mr Obama appears to making headway with Clinton supporters. The latest Associated Press-Yahoo News poll found that 53 per cent of the Democrats who favoured Mrs Clinton for the nomination two months ago now back Mr Obama for president.

That is an improvement from April, when only 40 per cent of Clinton supporters said they would back her rival of John McCain, the Republican candidate.

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