From a column by the late great scoop reporter Jack Anderson in the Ellensburg Daily Record on Sept 15, 1980 (elsewhere one can confirm that the specific date of the KKK endorsement was on July 30, 1980
Of course, it's not surprising Ronald Reagan would have been endorsed by the Klan. After all, just days after that initial Ku Klan Klan endorsement, Reagan, in his first major appearance after the Republican convention, went to the Neshoba County Fair in Philadelphia, Mississippi to talk about his belief in "state's rights." (a well-known code word for letting states segregate and such). Philadelphia, Mississippi of course, was known for only one thing: the murder of 1 African-American and 2 Jews working on voter registration by, you guessed it, members of the Ku Klux Klan.