We all know that SCOTUS, denying the Constitution by letting corporations trump people and money trump speech, made it possible for the US Chamber of Commerce to buy politicians in ways they had never hoped until now. But even with all that leeway, it seems that in their zeal to empower Republicans, they forgot to keep it legal.

With a war chest rivaling that of the Republican Party itself, the U.S. Chamber of Commerce has emerged in the last year as perhaps the Obama administration’s most-well-financed rival on signature policy debates like health care and financial regulation.

Critics on the left have long complained about the chamber’s outsize influence. But now they are taking on the business association directly, charging in a complaint filed Friday with the Internal Revenue Service that it violated tax codes by laundering millions of dollars meant for charitable work from a group with ties to the insurance giant A.I.G.

The complaint was brought by a group called U.S. Chamber Watch, which was created four months ago — with the strong financial backing of labor unions — to scrutinize the Chamber of Commerce’s growing influence and provide a counterbalance.

But chamber officials said they had complied with all tax laws and dismissed the complaint as a political ploy.

A chamber spokeswoman, Tita Freeman, said its opponents “are desperately looking for opportunities to undermine the chamber’s efforts to promote free markets and economic growth.”

The I.R.S. refused to comment on the complaint, citing the confidentiality of taxpayer records.

I.R.S. regulators have often been wary of wading into political grievances, particularly after evidence emerged during the Watergate scandal that the Nixon White House had sought to use the agency for political purposes… [emphasis added]