By Alina Selyukh
WASHINGTON (Reuters) - The outside "Super PAC" helping U.S. President Barack Obama's election campaign raised $4 million in May, according to a source familiar with the figures, marking its best month since it was created.
The group, Priorities USA Action, had struggled to catch up to its deep-pocketed Republican rivals in the money chase ahead of the November 6 election, but the source said June is already better than May in terms of the Super PAC's cash haul.
By comparison, Restore Our Future, the Super PAC political action committee backing Republican candidate Mitt Romney raised $5 million in May, its report filed with the Federal Election Commission showed.
Overall this campaign season, Restore Our Future has reported raising $61.5 million, while Priorities USA, the source said, has already put $20 million in the bank and has another $20 million in hard commitments.
Priorities USA Action's official filing is due to be presented to the FEC by the end of Wednesday.
Super PACs, spawned by a 2010 Supreme Court decision allowing outside groups to raise and spend unlimited amounts of money, are readying for probably the most expensive campaign in U.S. history. Outside spending has already topped $100 million.
Given the Obama's campaign's own fund-raising prowess, Democrats have been slower to embrace the notion of a Super PAC. Only in February did Obama announce a change of heart on his opposition to Super PACs, leaving Priorities USA with an uphill climb to convince the liberal wealthy of its crucial role.
In May, for the first time, Romney's campaign fundraising - donations subject to legal limits - topped Obama's, bringing in more than $76.8 million. Obama and his Democratic allies raised some $60 million in the same period.
Romney's Super PAC, Restore Our Future, came to prominence for funding attack ads which helped the former Massachusetts governor defeat rival Republican presidential hopefuls during the primary season.
In April, Restore Our Future raised $3.9 million compared to $1.6 million for Priorities USAA.
Restore Our Future had $8.4 million left in cash on hand at the end of May after spending $4.8 million on advertising, FEC filings showed.
Priorities USA has so far spent some $15 million on advertising, much of it in recent weeks on ads across six states hitting Romney on his private equity past and immigration positions.
The pro-Obama group plans to spend another $10 million on the air through the end of summer, the source said.
Restore Our Future, for its part, on Wednesday launched a $7.6 million ad spree in nine battleground states, picking on Obama's gaffe earlier this month that "the private sector is doing fine."
The group is one of several Republican outside spending groups helping Romney catch up and overtake Obama's campaign fundraising.
Much of Restore Our Future's cash in May came from first-time donors, including a $500,000 check from Arkansas billionaire investment banker Warren Stephens.
Another check of that size came from Pennsylvania healthcare executive and founder of Select Medical Rocco Ortenzio, whose total donation is now $750,000.
Three companies linked to Robert Brockman, an executive at Dayton, Ohio-based Reynolds and Reynolds, split a $1 million donations. The companies - CRC Information Systems Inc, Fairbanks Properties LLC and Waterbury Properties LLC - share a P.O. box with his Brockman's car dealership support company.
Notably absent from the filing is billionaire Las Vegas casino magnate Sheldon Adelson, whose $10 million pledge to the group this month is expected to be disclosed in July.
(Reporting by Alina Selyukh. Editing by Alistair Bell and Jackie Frank)