George Soros, the liberal billionaire investor who has largely laid low during the 2012 elections, announced $1.5 million in pledged donations Thursday to a trio of super PACs backing President Obama and congressional Democrats, PAC officials said.

Soros, who became nationally prominent in 2004 when he spent more than $20 million on independent groups supporting Sen. John F. Kerry (D-Mass.), will give $1 million to Priorities USA Action — which is supporting Obama — and $500,000 to Majority PAC and House Majority PAC, which are running ads on behalf of Democratic candidates for Congress.

The move could signal a softening of resistance by Soros and other wealthy liberals to contributing to super PACs and other independent groups that can accept unlimited donations. Soros had previously given $1 million to the American Bridge opposition-research group and smaller checks to other groups but had suggested that he was unlikely to get more directly involved.

Even with more than $2.5 million in total donations, Soros’s financial commitment pales in comparison with those of conservative donors such as casino magnate Sheldon Adelson, who has said he will give up to $100 million to groups supporting Mitt Romney and other Republicans this year.

Other top Republican donors include Houston home builder Bob Perry ($16 million) and Dallas billionaire Harold Simmons ($15.8 million). The top Democratic giver on record so far is New York hedge fund manager Jim Simons, who has given a combined $4 million to the same super PACs to which Soros just agreed to contribute.

Soros’s new gifts were announced at a Thursday lunch sponsored by a group of liberal donors called the Democracy Alliance, according to the New York Times, which first reported the pledge. The event was headlined by former president Bill Clinton and attended by other prominent Democrats, including House Minority Leader Nancy Pelosi (Calif.).

Soros’s gift follows the announcement by Chicago Mayor Rahm Emanuel this month that he was leaving his official perch at the Obama campaign to help raise money for Priorities USA Action.

Democrats have been worried about Republicans raising funds from wealthy conservatives, who have poured hundreds of millions of dollars into a network of groups aimed at toppling Obama. Many wealthy liberals, by contrast, have sat on the sidelines, in part because they say they are discomfited by the influence of big money in politics.