In playing with Google, we can see how the phrase "political knowledge" has been used in books over time. See below.
So the phrase "political knowledge" really took off in the 1950s, hitting its high point in the early 1960s, and then dropped. There were burps up in the 1970s (Watergate?) and the 1980s (Reagan?). It's odd the term never really appeared until the 1940s.
Interesting, while not being terribly informative.