By Alina Selyukh
WASHINGTON (Reuters) - The "Super PAC" backing Republican presidential hopeful Mitt Romney saw its best fundraising month yet in June, raising some $20 million and by far outpacing its chief rival group backing President Barack Obama.
Restore Our Future, the pro-Romney "super" political action committee, disclosed the cash haul on Monday but did not indicate how much remained in cash on hand at the end of June.
Half of the $20 million is expected to be cash from Las Vegas casino mogul and billionaire Sheldon Adelson, Republican Super PACs' most active donor this campaign season.
The pro-Obama Super PAC, Priorities USA Action, also had its best fundraising month in June but brought in $6.1 million.
The pro-Romney group, which is not allowed to coordinate with the official campaign efforts, had seen two months of tepid fundraising -- garnering $5 million in May -- as Romney himself kicked his campaign into full election gear and notably narrowed the money gap against Obama.
Springing to prominence during Romney's heated battle for the Republican nomination, Restore Our Future helped disarm several of Romney's party rivals like Newt Gingrich with multimillion-dollar ad attacks. Now, alongside other Republican groups, Restore has turned fire on Obama.
In June, the group spent about $15 million on ads, roughly half running in nine battleground states and another half booked in 11 states for the first week of August. The ads hit Obama for the state of U.S. economy and joblessness.
Super PACs, allowed to raise and spend unlimited funds, have become perhaps the biggest weapon in that fight. Operating outside of official efforts of campaigns and national parties, they have taken over much of the dirty work of negative advertising.
Obama's campaign has started sounding alarms that such outside spending groups would help Romney would overtake the president's fundraising -- traditionally an incumbent's advantage -- and make him the first sitting president to be outspent in an election.
Obama and the Democratic Party's early start still leaves him ahead of his Republican rival. The total he has raised for himself, the Democratic National Committee and the fund they use jointly stands at $552.5 million compared to Romney's $394.9 million raised for his campaign, the Republican National Committee and their joint fund. But Romney has garnered support of Wall Street and many wealthy Americans, disgruntled by what they see as Obama's anti-business rhetoric and policies.
Since Romney emerged as the presumed Republican nominee in April, he and the RNC outraised Obama and the Democrats for two past months in a row.
Super PACs are required to disclose all of their donors, and official spending and fundraising disclosures for June from them and campaigns are due on July 20.
(Editing by Cynthia Osterman)