The fierce struggle between Hillary Clinton and Barack Obama has lit a fire beneath fundraising efforts that have generated unprecedented sums totalling more than $12 million in the past 48 hours.

Mrs Clinton, who announced on Wednesday that she had been forced to lend herself $5 million (£2.5 million) to keep up with her rival, earned all that money back in record time by yesterday lunchtime. Aides – some of whom had begun working without pay – said that there had been a “tremendous outpouring of support” with 35,000 new donors pitching in after elections on “Super Tuesday”. The campaign has now doubled its initial target, of raising $3 million in 72 hours, to $6 million.

Mr Obama has received even more – at least $7 million – since voting stopped on Tuesday evening and appears on course to top the $30 million mark for the second month in succession. His campaign, fuelled by more than 400,000 small donors on the internet, has been moving ahead in the dash for cash in recent weeks amid signs that many, if not most, of Mrs Clinton’s benefactors have “maxed out” by giving the legal limit of $2,300.

She raised $13 million in January against Mr Obama’s total of $32 million. By comparison, John McCain, the Republican front-runner, received $39 million in the whole of last year.

The huge sums are needed for what has become the most expensive fight for the Democratic nomination in history. The two candidates have each burnt through more than $100 million so far, with much of the money going on political consultants, polling, private jets and TV advertising.

Mrs Clinton and her husband, Bill, have a vast network of wealthy supporters. They had expected to outpace all-comers in the money race and have been rocked back on their heels at the way that Mr Obama has matched her every inch of the way before overtaking her in recent weeks.

Mrs Clinton said that her $5 million loan was “my money”, thought to be from the fortune amassed with her husband – estimated to be worth as much as $50 million – since they left the White House. “We had a great month fundraising in January – broke all records,” she declared on Wednesday. “But my opponent was able to raise more money. We intended to be competitive, and we were – and I think the results [on Tuesday] proved the wisdom of my investment.”

For all her victories in the big states such as California and a new national Gallup poll showing Mrs Clinton with a 13-point lead over her rival, Mr Obama won more contests and split more than 15 million votes, securing 49.8 per cent support against 50.2 per cent for Mrs Clinton.

Yesterday Mr Obama embarked on a 2,600-mile sweep running diagonally from the southeast to the northwest of America, taking in stops in Louisiana, Nebraska and Washington state. All three hold nomination contests tomorrow that he is expected to win. He began in New Orleans, Louisiana, still recovering from the effects of Hurricane Katrina two years ago, where many of the largely black population blame President Bush’s Administration for not doing enough to help.

Mrs Clinton is focusing her immediate attention on Virginia, which votes next Tuesday and where she was attending her own rally yesterday.

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