* Restore Our Future spent $21 million in August
* Raised less than pro-Obama Priorities USA
* Romney relied heavily on outside spending groups
By Alina Selyukh
WASHINGTON, Sept 20 (Reuters) - The "Super PAC" backing Republican presidential candidate Mitt Romney plowed through $21.2 million in August as its fundraising declined for the second month, raising questions about how much of an ad-buying force it will be for Romney in the final weeks before the Nov. 6 election.
Financial disclosures filed on Thursday showed that Restore Our Future - the "super" political action committee whose attack ads on Romney's foes played a key role in boosting him to the Republican nomination - had just $6.3 million in cash at the end of last month.
The group's report to the Federal Election Commission said it raised $7 million in August - less than rival Priorities USA, a group supporting Democratic President Barack Obama.
Restore Our Future, which declined to comment, kicked up its spending in August to try to counter Obama's well-funded campaign, whose ads dominated the airwaves in many parts of the country last month.
Obama's campaign on Thursday reported spending some $83.2 million in August, when it raised $84.2 million.
Romney's campaign raised $66.1 million in August, but spent $61.2 million and could not dip into a large chunk of his cash because a provision in U.S. campaign law walled it off until after his official nomination at the Republican convention in Tampa in late August.
The law helps explain why Romney's team - despite no limits on contributions to its PAC and a healthy fundraising operation benefiting the Republican's campaign - wound up being stretched for cash at the end of August, and having to take out a $20 million loan to make it to the general election period.
The long-term impact of the dip in fundraising on Romney's overall effort is unclear.
Romney stands to benefit from other independent groups that can raise and spend unlimited amounts. They include American Crossroads and Crossroads GPS, a Super PAC and nonprofit co-founded by former George W. Bush aide Karl Rove, as well as Americans for Prosperity, which is funded by billionaire brothers Charles and David Koch.
Even so, the August totals - along with many of the independent Republican groups turning their focus to races for the U.S. Senate and House - could reflect some of the discontent that many conservatives feel toward Romney's campaign, which trails Obama's nationwide and in several of the politically divided "swing" states that will decide the election.
The academic Wesleyan Media Project last week found that 54 percent of pro-Romney or anti-Obama ads run between late April and early September came from independent groups.
Democratic groups, generally unable to match Republican groups' fundraising prowess this year, funded just 9 percent of ads benefiting Obama during that time.
The main pro-Obama Super PAC, Priorities USA, also spent heavily on the president's behalf in August, dishing out $9.5 million, which left it with $4.8 million in the bank.
But for the long-struggling Democratic group - Democrats generally disdain the unlimited fundraising of PACs - August marked its best fundraising month, with $10.1 million raised.
In the overall race for cash to keep voter turnout operations and advertising running through the election, Obama and his allies are still ahead.
The president and the Democratic Party have raised a total of just less than $740 million this campaign season, compared with roughly $630 million raised by Romney for his campaign and the Republican Party, according to news releases and disclosures.
In June the fundraising total for Restore Our Future, the pro-Romney PAC, raised $20.7 million, with $10 million of that coming from billionaire casino mogul Sheldon Adelson and his wife, Miriam.
In July, without any donations from the Adelsons, the group's fundraising total dipped to $7.4 million. The Adelsons also did not give in August.
Restore Our Future received two $1 million donations in August: one from Bob Parsons, billionaire founder of the web hosting giant Go Daddy; and from OdysseyRe Holdings Corp, a Stamford, Connecticut-based reinsurance firm owned by Toronto-based Fairfax Financial Holdings Ltd.
Corporations contributed just under half of Restore Our Future's cash last month.
Priorities USA got a $2 million gift from billionaire hedge fund manager James Simons and a total of $1.1 million from five different trade unions.