A fundraising laggard for much of the campaign, the political action committee (PAC) backing President Barack Obama raised more than a rival group backing Republican Mitt Romney, the second consecutive month the Democratic group has done so.

The pro-Obama group Priorities USA Action raised $15.2 million in September, compared with $14.8 million for the pro-Romney group Restore Our Future, according to new filings with the Federal Election Commission.

Restore Our Future, a formidable advertising force whose attack ads on Romney's Republican rivals helped him secure the party's presidential nomination, began the crucial month of October with $16.6 million in cash on hand.

American Crossroads, a Super PAC run by veteran Republican operative Karl Rove, on Saturday reported raising $11.7 million and kicking off October with $15.8 million in cash on hand.

DONORS GIVE BIG

Republican PACs have invested far more in advertising than Obama's backers, spending $84.1 million since September 10 alone, according to a Reuters analysis of disclosures through October 18. Democratic-aligned groups spent $23.1 million in that time.

Early this year, many Democratic donors were reluctant to donate to PACs supporting their party's candidates because they disliked the idea of contributing to attack ads and other negative advertising - a staple of spending by groups known as "Super PACs" because they have no limits on how much they can raise and spend.

In recent months, fundraising for Democratic PACs has accelerated to counter the waves of anti-Obama and anti-Democratic ads from Republican groups.

Priorities in September received $2 million from founder of Newsweb Corp Fred Eychaner; $1.5 million from billionaire hedge fund manager of Renaissance Technologies Corp James Simons; and $1 million each from film director Steven Spielberg, DreamWorks Animation SKG Inc CEO Jeffrey Katzenberg and New York lawyer David Boies Jr, according to filings.

The group had $7.3 million in cash on hand at the end of last month, Saturday's filing showed. Spending nearly three times less than Priorities in September, Restore Our Future started October with $16.6 million.

Restore's biggest donors in September included Texas home builder Bob Perry and billionaire Harold Simmons.

Perry gave $2 million in September and leads all donors with $10 million contributed to Restore Our Future during this election cycle. Simmons gave $500,000 for the month, increasing his total to the group to $1.3 million.

Simmons is also the biggest donor to American Crossroads. The Dallas patron was the group's biggest donor, giving $2.5 million in September. Together with his firm Contran Corp, Simmons has given $15.5 million to the Super PAC.

Its other big September donors included telecommunications conglomerate Armstrong Group of Pennsylvania, which gave $1.3 million, and Jay Bergman of PETCO Petroleum Corp in Illinois, who gave $1 million.

CONTROL OF CONGRESS

The Democratic PACs seeking to help candidates win House of Representatives and Senate seats also saw donations rise in September.

Majority PAC, the group helping Democratic candidates for the Senate, announced on Friday its biggest cash haul since its creation in 2010: $10.4 million in September, and another $9.7 million during the first week of October.

Its sister group, House Majority PAC, also had its best month in September and is on track to double that in October, an aide to the group said.

Democrats need a net gain of 25 seats to become the majority party in the 435-seat House, an outcome that most pollsters see as unlikely. Republicans need a net gain of four seats to take control of the Senate. Most pollsters also see that as unlikely.

Scores of Republican PACs and tax-exempt groups have been formed to support Republican House and Senate candidates. Tax-exempt groups do not disclose their finances.

American Crossroads alone spent $27.9 million on polling and ads, among other things, in September, filings showed.

Republicans showed greater fundraising strength at the party level in September.

The Republican National Committee raised $48.4 million and ended September with $82.6 million in cash on hand. Its Democratic counterpart took out a large loan.

The Democratic National Committee, which has focused on races for the Senate and House, raised $20.3 million in September and borrowed $10.5 million. It ended September $20.5 million in debt, according to its filing with the FEC.

A DNC representative declined to comment on the reason for the debt and how it might affect Democratic candidates in the final two weeks before the election. The party had $4.6 million in cash on hand heading into October.

The RNC reported $9.9 million in debt at the end of September, with its $82.6 million in cash on hand.

Unlike the DNC, the Republican Party has been working closely with its presidential candidate, Romney, who heavily relies on its grassroots operation. On the Democratic side, Obama's campaign has a huge grassroots organization itself and does not need such help from its party.

(Additional reporting by Alexander Cohen; Editing by David Lindsey, Bill Trott and Todd Eastham)

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