Mr Obama has received 258,000 donations since the start of the year

The Illinois senator's presidential campaign raised $32.5m (£16.2m) in the period from April to June, a record for a Democratic candidate, his team said.

But Mrs Clinton, whose team said she raised around $27m (£13.5m) in the same quarter, still leads in opinion polls.

Republican candidate John McCain has raised only $11.2m, his team said.

As a result, several top staff have taken pay cuts and some jobs will be lost in a campaign shake-up, Senator McCain's campaign manager told reporters.

His fellow Republican contenders have yet to announce their second quarter totals but are expected to do so well before the 15 July deadline for submitting reports to the Federal Election Commission.

Grassroots support

Mr Obama's fund-raising success will give added momentum to his campaign to become America's first black president, correspondents say.

Overall, he has raised over $58m (£29m) - of which the vast majority, $55.7m (£27.9m), is earmarked for his primary campaign. Some $31m of that came from his second quarter total, his campaign aides say.

DEMOCRATS' FUNDRAISING Barack Obama: $32.5m Hillary Clinton: $27m John Edwards: $9m Bill Richardson: $7m Christopher Dodd: $3.25m Totals as reported by candidates' campaigns; do not indicate how much is earmarked for primaries

Mrs Clinton, who leads in national and most state polls, is reported to have raised some $21m (£10.5m) for the primaries out of a total of some $27m in the second quarter.

Her first quarter efforts raised about $26m (£23m) in total, some $19m of which is earmarked for the primary battle. She has an extra $10m (£5m) on top, left over from her earlier Senate campaign.

She is due to campaign in the key state of Iowa later on Monday with her husband Bill Clinton, hoping to harness the former president's ability to wow the crowds and raise funds.

Former Senator John Edwards said his campaign had garnered a second-quarter total of just over $9m (£4.5m) from some 100,000 donors.

Fellow Democratic candidates Bill Richardson and Christopher Dodd reported raising $7m (£3.5m) and $3.25m (£1.6m) respectively.

Mr Obama said he had received at least 154,000 contributions in the last accounting period, bringing the total number of donors to 258,000 in the first half of the year.

This made his the "largest grassroots campaign" for this stage of a presidential race, he said.

"We now have hundreds of thousands of Americans who are ready to demand health care for all, energy independence and an end to the war in Iraq," Mr Obama said.

"That's the kind of movement that can change the special interest-driven politics in Washington and transform our country, and it's just the beginning."

Republicans

Mr McCain's disappointing showing will be a setback for the Arizona senator, once considered to be the frontrunner for the Republican nomination but now trailing his main rivals in many polls.

John McCain's campaign team now has little cash on hand

He had already announced an overhaul of his fundraising operation in April after declaring a disappointing $12.5m (£6.2m) for the first quarter of 2007.

Campaign manager Terry Nelson blamed a general disenchantment among the Republican faithful for the team's failure to raise more cash, which leaves them with only $2m on hand.

Mr McCain's support for the Iraq war and the immigration bill which failed in the Senate last week may also have cost him some support, observers say.

On the Republican side, former Massachusetts Governor Mitt Romney indicated on Friday that his second quarter funds fell short of the $21m (£10.5m) raised in the first quarter.

Former New York Mayor Rudolph Giuliani, who raised nearly $15m (£7.5m) in the first quarter, was expected to announce his figures on Monday or Tuesday.

Money pledged for the presidential election in November 2008 - should a candidate become their party's nominee - cannot be spent on campaigning for the primary.