By Alina Selyukh

WASHINGTON (Reuters) - House Majority PAC, a "Super PAC" aimed at helping elect Democrats to the House of Representatives, on Thursday said it raised $5.6 million in August in its best month yet.

The "super" political action committee had about $8 million left in cash on hand the end of last month, spokesman Andy Stone said. The group's fundraising last month exceeded its $4.3 million haul in all of the second quarter and by far beat out its paltry July result of $764,580.

House Majority PAC, like other outside Super PAC groups of both parties, can raise and spend unlimited funds. It is part of an alliance of Democratic groups formed largely to run ads to help the party withstand a multimillion-dollar offensive funded by Republican counterparts.

House Majority PAC has ads now running on TV in 12 races and has reserved another $22 million worth of air time covering a total of 40 congressional districts, Stone said.

Democrats need a net gain of 25 seats to become the majority party in the House, although pollsters see that as a highly unlikely outcome of the November 6 vote in which all 435 members of the House of Representatives are elected.

Republican groups, vowing to pour hundreds of millions into the races, hope to unseat Democratic President Barack Obama in the White House, keep the Republican majority in the House and take control of the Senate.

Super PACs' official finance disclosures for August are due to the Federal Election Commission by the end of Thursday.