In April, for example, the air traffic controllers’ union contributed $1 million to a pro-Obama super PAC. But other contributions are harder to trace. Last year, the American Federation of State, County and Municipal Employees gave $100,000 to Advancing Wisconsin, a tax-exempt group that supported labor’s fight with Republicans in that state; the donation was reported nowhere in Wisconsin, but it emerged in an annual financial report that unions must file with the federal Department of Labor.

Among the largest beneficiaries of corporate donations in recent years have been trade organizations like the U.S. Chamber of Commerce, which largely backs Republican candidates. As a nonprofit “business league” under the tax code, the chamber does not have to disclose its supporters, who helped finance its $33 million in political ads in the 2010 midterm elections.

But voluntary disclosures by corporations — usually at the prodding of shareholder advocacy groups — shed some light on the use of trade groups for lobbying or as pass-throughs for political spending. A search of voluntary disclosures, some collected by the Center for Political Accountability, which advocates for transparency in corporate political spending, found more than $6 million in chamber donations by 10 companies last year.

Two of the largest came from Prudential Financial and Dow Chemical, which each gave $1.6 million, while Chevron, MetLife and Merck each gave at least $500,000. Some of the donations were directed to the chamber’s Institute for Legal Reform, which lobbies for limits on liability suits.

Some contributions are disclosed by accident. Aetna’s check to the American Action Network, along with a $4.5 million contribution last year to the chamber, was mistakenly included in a filing with insurance regulators. The disclosure was first reported by SNL Financial, a trade publication. Even where companies pledge voluntary disclosure of political contributions, they often make an exception for donations to tax-exempt groups.

In 2007, Aetna signed an agreement with the Mercy Investment Program, a shareholders group, to disclose trade associations to which it made large contributions. On regulatory filings, the company initially described its $3 million contribution to the Chamber of Commerce as a lobbying expense, but the company now says it was intended to finance “educational activities.”

An Aetna spokesman would not say whether the chamber donation would appear on the company’s 2011 voluntary disclosure. Sister Valerie Heinonen, the director of shareholder advocacy for Mercy Investment Services, said that a failure to do so would violate the company’s pledge.