The Oscar-winning actor's big donation to Priorities USA Action, the "super" political action committee backing the president, was announced on Thursday and will be reported in the group's federal disclosures on Friday.

Priorities USA has raised $20.4 million this election cycle but lags far behind Restore Our Future, the Super PAC supporting Republican presidential candidate Mitt Romney. Restore Our Future has so far reported raising $81.5 million.

Super PACs are allowed to raise and spend unlimited funds. Operating outside of official efforts of campaigns and national parties, they have taken over much of the dirty work of negative advertising.

With four months to go before the November 6 general election, each side is on track to spend more than $1 billion on campaigning and ads.

Last month both Super PACs had their best month of fundraising yet. Priorities raised $6.1 million and Restore pulled in $20 million.

Overall, Obama's official campaign has raised more money than what Romney has raked in for his campaign.

But the Republican has made inroads with Wall Street and the business community, which have grown disenchanted with Obama's policies. Hollywood and the entertainment industry remain among the biggest groups supporting Obama.

"President Obama has done a remarkable job in historically difficult circumstances," Freeman, 75, said in a statement. "In return, he is the target of hundreds of millions of dollars from right-wing sources. I am proud to lend my voice - and support - to those who defend him."

While Obama is under fire for the flagging U.S. economy and stubbornly high joblessness, Priorities USA has spent much of its cash on ads skewering Romney for his role at private equity firm Bain Capital.

One of the first Hollywood celebrities to give to Priorities USA was comedian Bill Maher, whose $1 million contribution in February became controversial because of his politically charged jokes and comments.

Priorities has also received $2 million from Hollywood producer and DreamWorks Animation CEO Jeffrey Katzenberg, and $100,000 each from film director Steven Spielberg and comedian Chelsea Handler.

Freeman is in the Batman movie "The Dark Knight Rises" opening in theaters on Friday.

(Reporting by Alina Selyukh; Editing by Deborah Charles and Xavier Briand)

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