The University of Chicago Law School is a key. A reader writes:
The Law School is a notoriously conservative place, famous as the home of Chicago School of legal analysis (i.e., law and economics). While there are certainly progressive students and professors there, anyone at the law school would have to confront conservatism on a daily basis. As such, Obama had to hone his arguments to the conservative environment in which he taught. That is not to say that his time there turned him conservative - rather, it helped him to articulate his progressive arguments in such a way that he could convince conservatives.
Go check out his debates with Alan Keyes. Really riveting.