* Move could trigger more big checks from Democrats
* Soros once held record for campaign-year donations
By Alina Selyukh
WASHINGTON, Sept 27 (Reuters) - Billionaire financier George Soros has committed $1.5 million to "Super PACs" backing President Barack Obama and Democrats running for Congress in the Nov. 6 election, officials with those groups said on Thursday.
Soros - a prominent donor to liberal political causes - has pledged $1 million to Priorities USA Action, which is running ads to help the Democratic president's re-election bid, and $500,000 to Majority PAC and House Majority PAC, which help Democratic congressional candidates, the officials said.
The move by Soros, who had remained largely on the sidelines of this year's Super PAC fundraising barrage, could trigger more big checks from wealthy Democrats who previously avoided giving to the groups, citing dismay over the growing role of money in politics.
Super PACs are outside groups formally unaffiliated with campaigns. They have been raking in unlimited amounts from individuals, corporations and unions to support candidates or issues.
Until this year, Soros, whose net worth is estimated by Forbes at $19 billion, held the record as the biggest contributor in an election cycle for the $27.5 million he estimated he spent in 2004 to try to defeat Republican President George W. Bush.
His record was eclipsed this year by Las Vegas casino owner Sheldon Adelson, who says he has given $70 million to help Republicans in the 2012 election.
Soros' adviser said previously that the financier was more focused in this campaign on grass-roots political organizing.
Soros last year made six-figure donations to the Majority and House Majority PACs and pledged $2 million earlier this year to American Bridge 21st Century, a research-focused Super PAC that does no ads, and America Votes, a group that helps coordinate campaign work for liberal groups nationwide .
Soros' new pledges to the trio of Democratic Super PACs was revealed on Thursday at a gathering of big liberal donors known as Democracy Alliance, according to The New York Times, which first reported the commitment.
In August, Priorities USA for the first time raised more than its Republican counterpart that backs Mitt Romney in his bid for the presidency - $1 0.1 million to $7 million .
At the end of last month, Priorities USA had $4.8 million left in cash on hand, compared with $6.3 million left in the coffers of the pro-Romney Restore Our Future.
The Majority and House Majority PACs also saw fundraising increase in August. Democratic fundraisers said donors felt increased urgency in the last weeks before Election Day and in the face of powerful Super PACs backing Republican candidates.