An article from the New York Times News service attributes  Cheap Shot as originating in 1971 in a football context. 

https://news.google.com/newspapers?nid=2199&dat=19810125&id=mYwyAAAAIBAJ&sjid=uucFAAAAIBAJ&pg=2377,4227702&hl=en

And rapidly making its way to political commentary soon afterwards. 

(I'm on my mobile...  So I hope that link resolves well)