By Alina Selyukh and Alexander Cohen

WASHINGTON (Reuters) - Fundraising by Democratic "Super PACs" is off to a slow start for the 2012 campaign season, compared with aggressive efforts by Republican groups that plan to spend hundreds of millions of dollars in fights for congressional seats and the White House.

Three major Democratic groups said on Friday they raised a total of about $5.7 million in the first three months of the year, far behind rival Republican groups.

Majority PAC, focused on preserving Democratic control of the U.S. Senate, received $1.6 million in contributions in the first quarter of 2012, according to financial filings with the Federal Election Commission.

House Majority PAC, which aims to help Democrats regain control of the House of Representatives that they lost in 2010, received $1.5 million. American Bridge, a group that does research on rival Republicans, raised $2.6 million together with its non-profit arm.

The Democratic groups are working in tandem against several behemoth Republican groups such as American Crossroads, run by Karl Rove, a former top aide to president George W. Bush.

American Crossroads alone plans to spend more than $250 million on congressional and presidential campaigns for the November 6 election. Such independent Super PACs - which unlike campaigns have no limits on individual donations - will allow very wealthy donors to have a big say in shaping this year's elections.

Republican Super PACs, inspired by the emergence of the conservative Tea Party movement, led to painful losses for Democrats in 2010, when Republicans took control of the U.S. House.

The American Crossroads Super PAC reported having $23.6 million at the end of February, its FEC filings show.

American Crossroads also has a non-profit arm that is not legally required to disclose its fundraising to the FEC. According to tax forms cited on Friday by The Washington Post, the non-profit group had raised more than $76 million by the end of 2011 after launching in May 2010.

The three Democratic Super PACs had a total of $5.4 million in cash on hand at the end of March, their FEC filings showed.

"As the presidential primary and races up and down the ballot have unfolded, one thing is clear: Republicans and their outside groups are going to spend an unprecedented amount of money to advance their right wing ideology," the Democratic groups said in a joint statement on Friday. "We are confident we will provide a countervailing force to these extreme agendas."

LAGGING BEHIND REPUBLICANS

Many Democrats staunchly opposed the controversial 2010 Supreme Court decision that paved the way for Super PACs by lifting limits on how much independent groups could raise for political purposes.

Many Democrats' disdain for the court ruling - along with their dismay at the barrage of PAC-funded attack ads that seemed to define the bitter campaign for the Republican presidential nomination - put Democrats behind fundraising for PACs, analysts said.

Leaders of Democratic PACs say many potential donors, especially those who would make relatively modest contributions, have not realized the urgency being felt by groups that support Democratic candidates.

Another major Democratic PAC - the Priorities USA Action, the one Super PAC backing President Barack Obama - has gotten a slow start because until February, Obama did not support fundraising for the group.

Priorities had $2.8 million in the bank at the end of February, according to its latest filing. The group is due to report its March numbers on April 20.

REPUBLICANS STEP UP

The Republican National Committee, another engine of Republican Party fundraising for congressional races, on Friday marked its best fundraising month of the campaign season.

The committee, which has been digging itself out of a multi-million-dollar debt, said it raised $13.7 million in March, with debts declining by $1 million from February to $9.9 million at the end of last month.

The committee started fundraising jointly this month with Mitt Romney, the party's likely presidential nominee. It had $32.7 million in cash on hand at the end of March.

In February, the RNC reported receiving $10.7 million in contributions and $26.7 million in cash on hand.

Its counterpart across the party line, the Democratic National Committee, received $7.1 million in contributions in February, with $21.2 million left in cash on hand at the end of that month. The DNC had $5.8 million in debt.

The RNC and the DNC are also due to file their official March fundraising reports with the FEC on April 20.

(Editing by David Lindsey and Peter Cooney)