Updated

Liberal financial heavyweights are becoming more involved in the presidential campaign.

The growing list of financial supporters now includes George Soros, who has pledged $2 million to political groups supporting President Barack Obama's re-election, as well as progressive causes. Activists say the goal is to hold Republicans accountable and counterbalance the hundreds of millions of dollars that are expected to be spent supporting Mitt Romney and GOP candidates.

Soros' pledges include $1 million to the advocacy group America Votes and $1 million to American Bridge 21st Century, an outside "super" political committee supportive of Obama's campaign. So far, GOP super PACs have raised tens of millions of dollars more than their Democratic counterparts.

The Soros donations, which he announced to supporters by email Monday night, signal that wealthy liberals are becoming more involved in what is expected to be a costly presidential campaign. Some super PACs, like the Romney-supportive Restore Our Future, already have spent more than $50 million on TV ads.

Soros' contributions add to a small but growing list of wealthy progressive individuals and groups contributing to super PACs supporting the Democratic incumbent. They include comedian Bill Maher and Hollywood producer Jeffrey Katzenberg, who have each given more than $1 million to Priorities USA Action, a PAC founded by two former Obama aides. The Service Employees International Union has also donated $1 million to the group, which can't legally coordinate with Obama's campaign.

"As he has in the past, George is focusing his political giving in 2012 on grass-roots organizing and holding conservatives accountable for the flawed policies they promote," Soros adviser Michael Vachon said by email.

Soros, whose latest donations were first reported by The New York Times, contributed millions of dollars during the 2004 election to progressive causes. His support included more than $1 million to the liberal activist group MoveOn.org, which opposed President George W. Bush, and millions more to the group America Coming Together, which in 2007 was fined $775,000 for federal campaign-finance violations.

This presidential election is the first in which billionaires have a green light to give unlimited sums of cash to groups that support their favored candidates, thanks to a handful of federal court cases — including the Supreme Court's decision in the 2010 Citizens United case — that stripped away campaign-finance regulations of years past. Many of the new groups that have popped up have to regularly disclose their finances, but some super PACs' nonprofit affiliates do not, thanks to Internal Revenue Service rules.

Soros' contributions are a boost for groups like American Bridge, which has spent $4 million so far researching and tracking Republican candidates. Soros previously has supported America Votes, a Washington-based organization that helps progressive groups with political organizing.

Super PACs supporting Republican candidates and, in particular, Romney have pledged to raise hundreds of millions of dollars to compete against Obama's sprawling donor base. American Crossroads, a group founded by former Bush adviser Karl Rove, and its nonprofit arm have raised a combined $100 million.

Obama, meanwhile, has raised about $147 million so far. That doesn't count millions in additional contributions to the Democratic National Committee, which is helping Obama's re-election. Republicans have pointed to the built-in fundraising advantage Obama has as an incumbent for why GOP super PACs are necessary.

___

Follow Jack Gillum on Twitter: http://twitter.com/jackgillum