Source, in full
Attribution to Rockne sometime in the 1920s mentioned in Safire's Political Dictionary (2008) by William Safire, p. 401, but there is no definite mention of this yet located prior to the 1980s; in The Yale Book of Quotations by Fred R. Shapiro, a similar remark is credited to Arnold "Red" Auerbach, based on a 1965 citation in the Mansfield News Journal [Ohio]: "Show me a good loser, and I'll show you a loser."