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 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.

Democrats have been worried for months about Republican success in 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.