MANCHESTER, N.H. - An independent political action committee supportive of Newt Gingrich is planning to release a film critical of Mitt Romney's tenure at a private-equity firm, just days after a Las Vegas billionaire contributed $5 million to the group to bolster the former House speaker's White House run.

The Gingrich-leaning Winning Our Future PAC said on Sunday that the 28-minute video -- which assails Romney for "reaping massive awards" while head of Bain Capital -- will be posted online soon and could show up on TV ahead of several primary elections this month.

A person familiar with the development said Sheldon Adelson, a casino mogul and longtime donor to Republican candidates, made the contribution on Friday to Winning Our Future, which is run by Gingrich allies. The person, who was not authorized to discuss the matter publicly, said Adelson is expected to contribute to groups backing the Republican nominee, be it Gingrich or one of his rivals.

Both the film and the large contribution highlight the growing role that new "super" political action committees are playing in this election. Just weeks ago, a Romney-leaning super PAC called Restore Our Future hammered Gingrich with $3 million in negative ads that largely contributed to his eroding support before the Iowa caucuses. Gingrich finished in fourth place. The ads lashed Gingrich for his ties to federal housing giant Freddie Mac and for his reversal on issues such as climate change.

Now the tables have turned.

The film, called "When Mitt Romney Came To Town," assails Romney for "reaping massive awards" for himself and his investors. Bain has been credited with turning around dozens of companies, including well-known brands such as Domino's Pizza, but its record has been criticized -- especially by the Democratic National Committee -- for slashing jobs.

Rick Tyler, a former Gingrich aide who is now working for Winning Our Future, said the full video would be posted online "soon." Some segments could be used in shorter TV ads, he said, although there were no plans to run the full piece on television.

Super PACs have sprouted from a series of federal court rulings, including the Supreme Court's Citizens United case in 2010 that stripped away restrictions on corporate and union spending in elections. The groups can't coordinate directly with campaigns, but many of them active in this election are staffed by longtime supporters of the candidates.

While some super PACs have to disclose their contributors' names later this month, many will never be known. Some super PACs have established nonprofit arms that are permitted to shield contributors' identities as long as they spend no more than 50 percent of their money on electoral politics.