On September 25, 1965, Gaylord Parkinson, chairman of the California Republican Party, issued a rule for Republicans in the 1966 election campaign. It became known as the Eleventh Commandment: “Thou shalt not speak ill of any Republican.” Other wordings, such as “any fellow Republican,” are also in use.
Ronald Reagan, who ran for governor in 1966, popularized the rule. In his autobiography An American Life (1990), he wrote: “It’s a rule I followed during that campaign and have ever since.”