I found the following part of this NYT article quite interesting:

While the liberal organizations have been concentrating on members of Congress, Ms. Warren raised questions about the funding sources of policy groups like Third Way. Without specifically mentioning the group, she sent a letter on Wednesday to the chief executives of six of the country’s biggest banks asking them to reveal the groups they help finance.

Shareholders, she wrote, “have a right to know how corporate resources are spent, and, even more important, policy makers and the public should be aware of your contributions and evaluate the work of the think tanks accordingly.”