It's a similar construction to "fiscal conservative" or "social liberal", i.e. in this case people who had liberal views on racial issues in the 1940s-70s debat

It's a similar construction to "fiscal conservative" or "social liberal", i.e. in this case people who had liberal views on racial issues in the 1940s-70s debate over the civil-rights movement. I think this particular phrase is a bit idiosyncratic to Gottschalk, though. It's intended to give her a concise phrase that includes social liberals, conservatives who had liberal views on civil rights (common among northern conservatives at the time), etc., rather than having to constantly repeat a construction like "those who had liberal views on racial issues".