Conservatives who object to moderate Republican officeholders have over the years asked themselves how far they are prepared to go to rid themselves of their intraparty rivals. In the case of California’s senator Thomas Kuchel, the answer was: very far indeed. Kuchel (pronounced “kee-kul”) was among the last of the California progressives, politicians who had been decisively influenced by the middle-class, moral reform movement of the early twentieth century and leading progressive figures including Republican governors Hiram Johnson and Earl Warren. A key progressive reform, which was still in existence when Kuchel first was elected to the California State Assembly...