By Steve Holland and Alina Selyukh
WASHINGTON (Reuters) - U.S. Republican presidential hopeful Mitt Romney raised more than $100 million in June, his best month yet, a Republican official said on Thursday, putting pressure on President Barack Obama who could become the first incumbent to be outspent in a re-election run.
The run-up to the November 6 election is poised to be the most expensive election in U.S. history, as both campaigns rely on outside spending groups with no fundraising limits instead of public financing.
Romney's June haul was helped by the U.S. Supreme Court's ruling that upheld Obama's healthcare law - the Romney campaign has said it raised $4.6 million through 47,000 online donations in the 24 hours following the landmark ruling.
The Obama campaign has said it also raised a lot of money after the ruling but has not disclosed any figures.
Obama and his Democratic allies have not said whether they kept up with Romney in the cash stakes last month, but the Republican stunned them in May, when he outraised the president for the first time.
Despite good fundraising news, the former governor of Massachusetts has appeared to struggle in recent weeks.
Obama caught him flat-footed by focusing the campaign on immigration, knowing that the Republican has yet to work out an immigration reform policy, and then Romney and a key adviser gave mixed messages on whether they thought a key element of Obama's 2010 healthcare law would create a tax or not.
On Thursday, Obama campaign spokesman Ben LaBolt pegged the news of Romney's cash success in June as a distraction from negative coverage of the Republican.
"Americans are less concerned about how much money he raised to get himself elected and more interested in what he would do after repealing health reform, which he has refused to share, and why he won't disclose the necessary tax returns that prove whether or not he paid any U.S. taxes on his shell corporation in Bermuda," LaBolt said in a statement.
The Associated Press reported that an offshore company based in Bermuda has helped bolster Romney's wealth even though it did not appear on his state or federal financial reports for 15 years. The Romney campaign accused Obama's team of using the report to try to distract from the president's record on job creation.
ROMNEY OVERTAKES OBAMA
Obama's campaign set the monthly presidential fundraising record of $150.7 million in September 2008, but this year Romney, a former private equity executive, has tapped into discontent among business leaders at what they see as an anti-business agenda at the White House.
In May, Romney and his Republican allies raised more than $76.8 million, topping the more than $60 million Obama raised for his campaign and the Democratic National Committee.
That prompted a briefing by an Obama campaign official who predicted Romney would outdo the president in June with a $100 million fundraising report.
"I think Romney is going to continue to have big months," said the official, who spoke on condition of anonymity. "Combine that with the Super PAC (political action committee) stuff, you know, we are going to be the first incumbent outs pent. That's clear."
The activity of "Super PACs," allowed to raise and spend unlimited funds as long as they're independent of campaigns, have marked the 2012 campaign season by flooding airwaves with largely negative advertising.
The pro-Romney Super PAC, Restore Our Future, on Thursday revealed a purchase of $7.2 million worth of ad time across 11 battleground states during the airing of the Olympic Games, from July 31 through August 9.
The campaigns and Super PACs are due to officially disclose their June fundraising and spending on July 20.
(Additional reporting by Jeff Mason; Editing by Paul Simao)