While super PACs emerged as political players in the 2010 congressional elections, those groups were outspent by non-profit organizations -- many of which keep their donors' identities secret, a new report finds.

More than 100 "social welfare" non-profits spent about $95 million on political expenditures in that election, compared with $65 million by super PACs, according to a joint investigation by the Center for Public Integrity and the Center for Responsive Politics, which track campaign money. Nearly 90% of the non-profit spending came from organizations that never publicly disclosed their donors, the analysis found.

The 2010 election marked the first time super PACs played a role in federal contests. Unleashed by a pair of federal court rulings, super PACs can accept unlimited donations from corporations and unions to run ads calling for the election or defeat of federal candidates. They must disclose their donors' names, but several super PACs maintain non-profit arms that do not disclose the sources of their funding.

In the 2012 election cycle, super PACs are outspending non-profits, the analysis found. Super PACs have spent $120 million, compared with $9 million by non-profit groups, known as 501 (c)(4)s, for the section of the tax code under which they are organized. The investigation's author, Michael Beckel, notes that could change.

Casino mogul Sheldon Adelson, for instance, has suggested he would give $35 million to three Republican-aligned non-profit groups, The Huffington Post reported over the weekend. Adelson and his wife, Miriam, have donated $20 million to Newt Gingrich's failed bid for the Republican presidential nomination and reportedly have contributed $10 million to a super PAC aiding the GOP's presumptive nominee, Mitt Romney.