Top U.S. corporations quietly bankrolled politically active nonprofits to the tune of at least $185 million in a single year, the Center for Public Integrity reports.

Based on an analysis of voluntary disclosures filed by the three hundred largest public companies, most in 2012, the center found that more than a thousand 501(c)(4) social welfare groups and 501(c)(6) trade associations received corporate contributions, including more than two dozen that received at least $1 million. Roughly 84 percent of the $185 million in self-reported funds went to trade associations like the U.S. Chamber of Commerce, America's Health Insurance Plans, and the American Petroleum Institute, while about 13 percent was collected by so-called social welfare groups and the remaining 3 percent went to other politically active entities, including the Congressional Black Caucus Foundation, the National Conference of State Legislators, the Heritage Foundation, and the American Legislative Exchange Council.

According to CPI, the U.S. Chamber of Commerce, a vocal opponent of more robust corporate disclosure, was the most frequently identified beneficiary of contributions from the Fortune 300 companies included in the study, receiving a total of nearly $11 million from at least sixty-two corporations. In 2012, the chamber reported spending more than $32 million on political advertising.

The $185 million figure does not include contributions from some of the nation's largest companies — including Wal-Mart Stores, ExxonMobil, and AT&T — which do not voluntarily disclose their political contributions. Others, such as Best Buy, Delta Air Lines, and Google listed recipients but not the amounts contributed. Among those that disclosed their contributions to politically active nonprofits, the biggest donor was Exelon Corp., the nation's largest nuclear power plant operator, which gave a total of $26.2 million in 2012 to two dozen nonprofits, including $250,000 to the Chamber of Commerce. Only two other companies reported giving more than $10 million to 501(c)(4) and (c)(6) organizations — health insurer WellPoint, which contributed $19 million in 2012 and 2013, and Microsoft, which contributed $12.5 million between July 1, 2011, and June 30, 2012.

"Companies increasingly see disclosure as good governance," said Bruce Freed, president of the Center for Political Accountability, which advocates for greater corporate self-reporting and each year grades major U.S. companies on their policies and practices. "There is a premium on transparency. Openness on this is in their self-interest."