MIAMI: US President Barack Obama, renowned as a champion fundraiser who piled up $750 million in 2008, warned supporters Tuesday he would be outspent by his foe Mitt Romney in this year's election.

The president lashed the Republican for huddling last weekend at a “secretive retreat” for rich donors and chiefs of fundraising committees targeting Democrats, portraying his policies as harmful to the middle class.

“I will be the first president in modern history to be outspent in his re-election campaign, if things continue as they have so far,” said Obama in a fundraising email sent out to his supporters.

“We can be outspent and still win—but we can't be outspent 10 to 1 and still win.” Obama's email came as he wound up a two-day campaign and fundraising swing through New Hampshire, Massachusetts, Georgia and Florida, reportedly set to raise up to $5 million dollars for his election war chest.

In one event on Tuesday, Obama appeared in the plush Miami beach residence of a telecommunications executive and his wife, as around 30 guests chowed down on cuisine from a top chef after paying $40,000 per head to attend.

Since clinching the Republican nomination, Romney has set about building a formidable fundraising machine, which he will use for a torrent of political ads and get out the vote efforts.

His haul, which some pundits think could hit up to $800 million will erase the huge money advantage Obama enjoyed over the Republican candidate in the 2008 general election race, John McCain.

In May, Romney overtook the president in monthly campaign fundraising figures, piling up $76 million dollars over the month, compared to the more than $60 million by the incumbent Democrat.

Romney is also set to reap an important advantage from allied big money political action committees known as Super-PACS, which thanks to a controversial Supreme Court ruling can raise unlimited funds.

The Obama campaign, which argues that Romney would favor rich friends and big business in his policies as president to the detriment of the middle class also raises alarm about the identity of the Republican's donors.

“The Romney campaign raises more than we do, and the math isn't hard to understand,” Obama said in the email.

“We raised almost three-quarters of our money from donors giving less than $1,000, while Mitt Romney's campaign raised more than three-quarters of its money from individuals giving $1,000 or more.”

Obama pulled in far more individual donors, however, at 572,000, with 98 per cent of them giving less than $250.

The president told supporters in Miami on Tuesday that Romney would use his cash bonanza to distort his record in what is shaping up as a knife edge election on November 6.

“The other side will spend more money than at any time in United States history on negative ads that have a very simple message—that say the economy is bad, and it is all my fault.”

“I've got other responsibilities I'm attending to,” Obama wrote.

Obama's email was part of a blizzard of money shots fired out by campaigns as the latest end-of-month fundraising deadline loomed.

Campaigns compete fervently to top one another in month by month fundraising, as their figures are seen by the media and pundits as a key metric of the state of the race and the appeal of candidates.

In other appeals on Tuesday, First Lady Michelle Obama told supporters they had one last chance to offer a donation and have the chance to take part in what have become regular dinners for supporters with her and her husband.