A group of Aetna investors has written the insurance company’s CEO to allege that Aetna violated its own political disclosure policy when it gave $7.8 million to the nonprofit American Action Network and the U.S. Chamber of Commerce.

“We believe Aetna is not in compliance with its corporate political and lobbying disclosure policy, a policy which we negotiated and expected would be met in spirit and in letter,” read the Monday letter to Aetna CEO and President Mark Bertolini from Mercy Investment Services Inc. and the Sisters of Charity of Saint Elizabeth, two Catholic groups with investments in Aetna.

The letter cites a spate of news reports following the disclosure last month by SNL Financial that Aetna had donated $3.3 million to AAN, a conservative advocacy group, and $4.5 million to the chamber, two organizations that have spent heavily on advocacy and campaign-style ads. The letter also points to reports that Aetna donated millions to America’s Health Insurance Plans that was transferred to the chamber to oppose enactment of the 2010 health care law.

Last month’s disclosure of Aetna’s payments to ANN and the chamber also drew a letter of complaint to the IRS and a letter to Bertolini from Citizens for Responsibility and Ethics in Washington, which called on the insurer to stop using corporate funds to influence elections. CREW also requested that the company publicly report its activities if it “persists in such spending.”

Aetna officials could not be reached for comment. But in a letter responding to CREW last month, Bertolini stated that Aetna fully complies with all federal and state disclosure requirements and publishes an annual report on its political contributions and related activity. The company has corrected “an error” in its filings to the National Association of Insurance Commissioners to indicate that the money was not for lobbying activities but for “educational activities,” Bertolini wrote. (The NAIC filings were the basis for the SNL Financial report.)

But in their recent complaint to Aetna, the Catholic investors point to a 2007 letter of agreement in which Aetna promised shareholders that it would disclose all expenditures for lobbying and political purposes, as well as trade association payments and grass-roots spending. The Aetna policy followed a 2006 shareholder resolution calling for the company to disclose its political spending.

“We, investors, withdrew the resolution in good faith expecting that the resolution establishing oversight and transparency would be followed, revised as best practices evolved and in place for reference by the members of the committee preparing the annual reports,” read the letter.

In an interview, Sister Valerie Heinonen, one of the letter’s authors, said investors were “dismayed” that the agreed-on policy had not been followed.

“We negotiated a policy, and it apparently was ignored or not followed,” said Heinonen, director of shareholder advocacy for Mercy Investment Services Inc., a pooled investment fund of the Institute of the Sisters of Mercy of the Americas, a Catholic order.

“Certainly, everybody has the right to make these kinds of corporate contributions,” Heinonen added. “However, as investors we wanted to know what corporate money was being spent.”

The letter comes on the heels of a spate of shareholder resolutions around the country this spring calling for full disclosure of corporate political expenditures. Corporate donations to politically active trade groups and nonprofits are drawing shareholder scrutiny in the wake of the Supreme Court’s 2010 ruling to deregulate political money.