The NY Times offers this must-read extensive report. A snippet:

These large donations — none of which were publicly disclosed by the chamber, a tax-exempt group which keeps its donors secret — offer a glimpse of the chamber’s money-raising efforts, which it has ramped up recently in an orchestrated campaign to become one of the most well-financed critics of the Obama administration and an influential player in this fall’s Congressional elections.
They suggest that the recent allegations from President Obama and others that foreign money has ended up in the chamber’s coffers miss a larger point: The chamber has had little trouble finding American companies eager to enlist it, anonymously, to fight their political battles and pay handsomely for its help.
And these contributions, some of which can be pieced together through tax filings of corporate foundations and other public records, also show how the chamber has increasingly relied on a relatively small collection of big corporate donors to finance much of its Washington agenda.