WASHINGTON (Reuters) - Mitt Romney had his best fundraising month yet in March, but the presumed Republican nominee remained far behind Democratic President Barack Obama in the money game, filings with the Federal Election Commission showed on Friday.
Romney reported raising nearly $12.7 million last month, with some $10.1 million left in the bank as he sought to wrap up the nomination and focus on raising money for the costly fight to unseat Obama in the November 6 election.
The deep-pocketed "Super" political action committee, or PAC, that supports Romney, Restore Our Future, brought in $8.7 million in March, with three-quarters of it remaining on hand.
Democratic PACs have struggled to match the prowess of Restore Our Future, but Obama's campaign - spared the strenuous nominating process - raised $34.8 million in March and had $104 million left in the bank at the end of the month.
Republicans and Democrats are waging a war to raise as much money as possible to fund crucial television and radio airtime and get-out-the-vote efforts, projected to total $1 billion on each side of the aisle.
Romney's March haul came at a time when the former Massachusetts governor was still facing stiff competition from his last big conservative challenger, Rick Santorum, who has now dropped out of the race.
The pro-Obama group Priorities USA - a Super PAC that can raise and spend unlimited funds but cannot coordinate with the campaign - reported raising $2.5 million in March. It spent only a fraction of it, $318,254, and had $5 million left on hand.
While Romney just started fundraising jointly with the Republican National Committee this month, Obama has been taking full advantage of that option with his party organization, the Democratic National Committee since last year.
The joint fund greatly increases how much a donor can give to help a candidate, thanks to larger contributions allowed for party organizations in addition to the campaign.
Obama's two joint funds shared with the DNC had about $6.5 million in cash on hand and sent $7.5 million to the Obama campaign and $10.5 million to the DNC.
Both the RNC and the DNC plan to spend nearly everything they raise to help their candidates.
The RNC in March also had its best month, raising $13.7 million and reporting $32.7 million in cash on hand, according to the FEC filings. The DNC reported receiving a total of $17.9 million in March, with $24.4 million in the bank.
But donations to outside Democratic groups have lagged those given to Republicans, a source of concern for Democrats.
Campaigns can take only $2,500 from each donor, once for the primary process and again for the general election. Super PACs can take unlimited donations as long as they do not coordinate with the campaigns, and these have largely taken over the dirty task of negative advertising.
The pro-Obama Super PAC, Priorities USA Action, has struggled to catch up to the pro-Romney PAC Restore Our Future.
Restore Our Future on Friday reported raising $8.7 million in March, spent $12.7 million on knocking Santorum out of the race but still emerged with $6.5 million on hand.
The group's biggest donors last month included Texas billionaire banker Harold Simmons, who gave another $600,000 for a total of $800,000; hedge fund manager Kenneth Griffin of Citadel LLC, who gave $850,000 for a total of $1.1 million; and Huron Carbon LLC, which gave $1 million and shares an address in Florida with Oxbow Carbon, run by Bill Koch and itself a big donor to the Super PAC.
Other big donors included Charles Schwab Corp founder Charles Schwab and his wife, Helen, Cisco Systems Inc Chairman and Chief Executive John Chambers, New Balance Athletics Chairman James Davis, Marriott International Inc CEO J.W. Marriott Jr. and his brother Richard Marriott, chairman of Host Hotels and Resorts Inc.
Priorities received $1 million from Amy Goldman, heir to the fortune of New York real estate mogul Sol Goldman, and a second installment of $500,000 from Kareem Ahmed, CEO of California medical billing and collecting firm Landmark Medical Management.
The PAC also received $250,000 from Anne Earhart, granddaughter of oil baron J. Paul Getty. Earhart is also the top donor to another Democratic Super PAC, opposition research-focused American Bridge.
Other big donors included U.S. standup comedian and talk show host Chelsea Handler and longtime Democratic donor New York investor Bernard Schwartz, both of whom gave $100,000.
Helping Romney bridge the gap between his money power and Obama's is American Crossroads, perhaps the most formidable Republican Super PAC, which was co-founded by Karl Rove, former top aide and election strategist for President George W. Bush.
American Crossroads and its non-profit sister group Crossroads GPS have made plans to spend up to $300 million on this election cycle and by the end of March were two-thirds of the way to raising that sum, having hauled in $99.8 million over 2011 and the first quarter of 2012.
American Crossroads reported $24.4 million left in cash on hand, but raised only $1.2 million in March. Citadel investor Griffin was its top donor with his $700,000 contribution.
The non-profit Crossroads GPS is not legally required to report its fundraising or donors to the FEC.
The groups have been running ads slamming Obama, his policies and his party's congressional candidates. The $300 million plan covers the efforts launched last year into the rest of the cycle, with two-thirds focusing on the presidential race and the rest on Senate and House of Representatives races.
One of Romney's two rivals left in the Republican race is Texas Representative Ron Paul. His campaign's FEC filing on Friday showed he raised $2.6 million and had $1.8 million left in cash on hand at the end of March.
The second rival, former House Speaker Newt Gingrich, was heavily in debt, running up a $4.3 million tab. He raised $1.7 million and spent about $2 million in March.
The Super PAC backing Gingrich, Winning Our Future, has been heavily reliant on Las Vegas casino billionaire Sheldon Adelson and his family. In March, despite Gingrich's campaign fading far into Romney's shadow, Adelson's wife, Miriam, gave another $5 million, bringing the total Adelson clan contribution to the Super PAC to $21.5 million.
Santorum, who ended his campaign on April 10, also had nearly $2 million in debt and has continued to try to raise money to pay for it. He raised almost $5 million in March and spent $5.8 million.
The Super PAC backing his candidacy ended the month with less than $270,000 on hand, having spent almost all of the $8.3 million it had raised over the campaign cycle. Foster Friess, Wyoming millionaire investor and Santorum's friend, gave another $500,000 to the PAC in March for a total of $2.1 million.
Additional reporting by Steve Holland in Scottsdale, Arizona; Editing by Todd Eastham and Peter Cooney