A progressive watchdog group shaping Senate Democratic-led proposals targeting the private equity industry fills its war chest with millions of dollars from donors who get rich off lucrative investment funds, documents show.

Private Equity Stakeholder Project, a group based in Chicago, has established itself as one of the top outside allies of Sen. Elizabeth Warren (D-MA) and other lawmakers launching investigations “to hold private equity accountable” for “sucking money out of the rest of the economy.” In recent years, senators have cited PESP in press releases unveiling corporate crackdown bills, while the group’s research has helped form the basis of congressional letters demanding information from companies.