Mitt Romney has out-raised US President Barack Obama for the first time in this year’s White House race, with the challenger raking in more than $76 million, campaign figures showed today.

Mr Obama, who proved himself a fundraising juggernaut during his presidential run four years ago, raised "more than $60 million" in May along with the Democratic National Committee and local committees, the campaign said in a Twitter message.

That is a monthly record for the president's 2012 re-election bid, but the fact that Mr Romney shot past him on the fundraising front suggests the dash for campaign cash over the coming months will be fiercely competitive ahead of November's polls.

It also shows Mr Romney is unmistakably on a roll since becoming the all-but-certain nominee more than one month ago.

Mr Romney's camp said he and the Republican National Committee raised $76.8 million in May, and they have $107 million in cash on hand.

It also was quick to suggest Democrats might be turning on Mr Obama and donating to Mr Romney.

"Whether (Romney donors) are Republican, Democrat, Independent, a first time political donor or a former Obama donor, this is not just a campaign; it's an opportunity for the country," Mr Romney's finance chairman Spencer Zwick said in a statement.

"It is clear that people aren't willing to buy into 'hope & change' again," he added, referring to an Obama campaign slogan from 2008. "Voters are making an investment because they believe that it will benefit the country."

Polls show a tight race, with many putting Mr Obama just a few percentage points ahead of his challenger in national surveys, often within the margin of error.

Mr Romney's campaign said 93% of the May donations - more than 297,000 of them - were of $250 or less, and came from all 50 states and the capital Washington. Those small donations raised $12 million out of the total.

Mr Obama pulled in far more individual donors, however, at 572,000, with 98% of them donating less than $250.

More than 2.2 million people have donated to Mr Obama's re-election bid, the campaign said on his official Twitter feed.

The president raised $43 million in April, a drop from the $53 million he raised in March, while Mr Romney brought in $40.1 million in April, nearing parity with Mr Obama.

These monthly amounts do not, however, take into account contributions from political interest groups called "super-PACs," which can raise and spend unlimited funds to support candidates, although they cannot directly fund a campaign.

Experts say Mr Romney appears to be benefiting more from the new rules, with wealthy conservatives funnelling huge sums into outside groups that support his agenda.

Fundraising is vital to US presidential elections, when candidates criss-cross the country for months and roll out several advertising campaigns, spending hundreds of millions of dollars in the process.