WASHINGTON — 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. Some liberal celebrities, like comedian Bill Maher, have given $1 million so far this year to support Obama.

"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.

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.

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 $147 million through April 30. 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