Well, Thurgood Marshall was obvious. But look deeper. The other judge Kagan clerked for, Abner J. Mikva. Mikva is a supporter and member of the advisory board of the very Progressive American Constitution Society.

From the American Constitution Society website: "ACS is engaged in a multi-year initiative, the Constitution in the 21st Century, to promote positive, much-needed change in our legal and policy landscape. The project brings together scholars and practitioners to formulate and advance a progressive vision of our Constitution and laws that is intellectually sound, practically relevant and faithful to our constitutional values and heritage. The centerpiece of the project is a series of issue groups focused on discrete areas of law and policy, through which a wide range of members will develop, communicate and popularize progressive ideas through papers, conferences and media outreach."

http://www.acslaw.org/c21