Now the chamber’s political arm is turning to the November elections, and it expects to spend $50 million or more to push pro-business candidates, usually Republicans. As part of a wave of new commercials broadcast this week, the chamber’s California affiliate attacked Senator Barbara Boxer  a Democrat running for re-election against Carly Fiorina, the former chief executive of Hewlett-Packard  and accused Ms. Boxer of “destroying jobs” by voting against business.

Cyrus Mehri, a Washington lawyer who brought the I.R.S. complaint on behalf of U.S. Chamber Watch, said in an interview that the chamber’s current political activities were, in effect, being underwritten with money intended for charitable work.

The complaint focuses on loans and grants totaling about $18 million that were made beginning in 2003 to a nonprofit affiliate, the National Chamber Foundation, by the Starr Foundation, a charity started by the founder of A.I.G. and now led by Maurice R. Greenberg, the insurer’s former chairman.

Lawyers for Chamber Watch said their research, based largely on public tax filings, found that none of the principal on some $12 million in loans had been paid back and that the money appeared to have been given to the chamber’s foundation for unrestricted use.

The lawyers said that the money, in violation of nonprofit restrictions, was ultimately funneled to the chamber itself and used to finance broader political causes, including support for legal tort reform to shield companies like A.I.G. from liability. Mr. Greenberg himself had worked to promote restrictions on lawsuits, the complaint notes.

“You have millions of dollars improperly going to the chamber,” said Mr. Mehri, who drafted the complaint with Gail M. Harmon, a Washington lawyer specializing in tax law. “This is not a technical violation.”

But Stan Harrell, chief financial officer for the Chamber of Commerce, said in an interview Friday that the chamber’s lawyers and its accountants at Ernst & Young had reviewed the Starr Foundation funding and found that it complied with all relevant tax law.